Depending on the app, you can sometimes use more than one app at once. You can also watch a video in a small window while you do other things.
Drag up from the bottom of the home screen, hold, then release.
In the list of recent apps, tap the app's icon at the top of the preview to see 

Be sure to tap the app's icon instead of the screen preview, which opens the app.

Drag the divider line all the way up or down.

With some apps, you can watch video in a small, movable frame while you do other things. Use it when navigating with Google Maps, too.
When you’re playing a video or navigating with Maps, go to the home screen or app you want to use.
The video (or navigation) shrinks.
You can drag the video (or navigation) anywhere on the screen.

