Unless you are using an older iPhone, I suspect that you are using an iPhone that can support 3D Touch. This is a gesture similar to tapping, except that you push down a little bit more. 3D Touch was introduced with the iPhone 6s in September 2015, and also works on the iPhone 7, iPhone 8, and the iPhone X (and the Plus variants of those phones). But even though 3D Touch has been around for many years, I talk to many folks who don't even know that the feature is there. Frankly, I forget about it sometimes too. But there are tons of really useful things that you can do with 3D Touch. Here are a few of my favorites.
Quickly jot something down
I often need to quickly jot something down, like a phone number, a name, a case number, etc. The built-in Notes app is a great place to put that information. Of course, you can open the app and then tap on the button to create a new note, but it is faster to use 3D Touch. Just push down on the app icon on your home screen and tap New Note.
Perhaps even more useful is the option just below that: New Checklist. If you need to jot down a number of items, such as a grocery list, the New Checklist option after you 3D Touch will open the Notes app, create a new note, and then enter the checklist mode (normally accessed by pressing the icon of the check mark inside of a circle). Using 3D Touch and tapping New Checklist is far, far faster that doing all of those steps one at a time.
Compose a new email, without distractions
If you 3D Touch on the built-in Mail app icon, there is a New Message option. Thus, using 3D Touch is a fast way to compose a new email. But the real reason that I like this shortcut is that whenever I open the Mail app to compose a new email, the first thing I see when the Mail app opens is a list of emails, which probably includes some new ones that I haven't seen yet. Thus, I find myself distracted, and sometimes sidetracked, by those messages. By the time I start composing my email, I may have even forgotten what I was going to say. When I use the 3D Touch shortcut to compose a new email, I don't see my Inbox until my new email is composed and sent.
3D Touch cursor
When typing an email, or when typing virtually any other text, if you push down on the keyboard, the keys turn blank and the keyboard turns into a trackpad. You can slide your finger around to move your cursor up a few lines to edit or add to text. Not only does this save you the trouble of tapping to select a new location for the cursor, I also find that it is far more precise than just tapping on text you previously typed.
While you are moving the cursor around, you can 3D Touch again to select a word, and then drag your finger to select multiple words.
Message a specific person
If you tap on the Messages app icon, you will probably see your most recent text message conversation. But if you 3D Touch on the Messages app, you will see a list of names of folks who have recently had text message conversations with you. Assuming that you wanted to send a message to, or review a recent message from, one of those three people, this is a faster way to jump directly to the text message conversation with that person.
Beware of Contacts
This isn't as much of a tip as it is a warning. If you 3D Touch on the built-in Phone app, you see a list of four favorites. Tap a name, and you call that person. That makes sense. What I don't like is that if you 3D Touch on the Contacts app icon, you see that same list of Phone favorites, and tapping one of those names will also call that person. That shortcut makes sense to me on the Phone app icon, an app used to call people, but not on the Contacts app icon. It would make much more sense to me for a 3D Touch on the Contacts app to bring up the Contacts entry for that person so that you can review contact information. And that might be the behavior that you were expecting as well, which can cause quite a surprise if you were intending to quickly bring up a person's contact information to see some detail about the person and instead you find yourself calling that person's phone.
Mark my location
If you 3D Touch on the Maps app icon, the first choice is to Mark My Location. Tap this to drop a pin on the map at your current location. This can be useful if you are parking a car or a bike and then you are going to walk somewhere else and you are worried that you might forget where your car or bike was located.
Bluetooth and Wi-Fi
I frequently have a need to open the Settings app and go to the Wi-Fi settings or the Bluetooth settings. Both are located near the top of the list after you open the Settings app, but an even faster way to access these settings is to 3D Touch on the Settings icon and then tap Wi-Fi or Bluetooth.
Get this app first
Sometimes I want to start using an updated version of an app, so I will open the App Store icon, tap Updates, pull down from the top of the screen to see what updates are available, and then I'll tap the button to update all of my apps. Normally this works fine, but sometimes I find that I really want my iPhone to start by updating app X, and instead my iPhone is slowly updating apps Y and Z. As I wait, I cannot even launch the app that most interests me because the app icon is gray. Ugh.
To solve this, 3D Touch on the app icon on your home screen for the app in question, and then tap Prioritize Download. This will tell your iPhone to put the other updates to the side and immediately start updating this app.
Speaking of the App Store, you can 3D Touch on the App Store icon to see a few choices, one of which is Search, which brings you directly to the search function of the App Store.
Adjusting 3D Touch
You can adjust how hard you need to press on the display to trigger 3D Touch. Open the Settings app and go to General -> Accessibility -> 3D Touch to select Light, Medium or Firm. You can also turn off 3D Touch, if for some reason you need to do that.
And much more
This is just the tip of the iceberg when it comes to 3D Touch. For example, you can also use it to "peek" at a link or a file before you officially open it. And many third-party apps offer interesting 3D Touch options, such as the Launch Center Pro app which lets you see miniature icons. Click here to see a short video by Apple showing off the features of 3D Touch.
If you are looking for something to do while in waiting in line at the grocery store, on a train, etc., take a few minutes to play around with 3D Touch in different places to find other interesting uses. 3D Touch is a useful, but I suspect underused, feature on the iPhone.