Active sessions
Active sessions lists every browser signed in to your account, with its IP address and when it was last seen. This device is the one you're using.
Select Sign out next to a session to end it. That device has to sign in again.
Sign out everywhere else
Select Sign out everywhere else. It signs out every other browser and the apps kept signed in, and forgets remembered browsers. Use it when you lose a device or see one you don't recognize, then change your password.
Sign out of one app
Under Signed in to, select Sign out of next to the app. You stay signed in here, so opening the app again signs you straight back in. Some apps can't be signed out from here; the list says so.
Apps kept signed in
Apps & devices lists apps you chose to keep signed in. They stay signed in until you select Sign out, sign out in the app, or change your password.
Remembered browsers
Browsers where you ticked Remember this browser for 30 days. Select Forget to make that browser ask for your code again. Forgetting doesn't sign it out.