Your smartphone wouldn't be all that useful without all the apps you've downloaded. Even when you're not actively poking around within a program, it can run in the background—updating your location, checking your email, or playing music—to make life more convenient. But to do so, it needs permission from your phone.
Permissions let Google Maps check where you are in the world, your camera app peek through the phone's camera, and your favorite messenger scan your contacts before sending an SMS. Typically, apps request this type of access when you first open them. But you might end up granting some of them permissions that go beyond what the apps actually need. To check on these, you should regularly audit your app permissions.