A session is one browser that is signed in to your account. My profile → Sessions lists every session that is still active, so you can spot one you don't recognise and sign it out.
Click Sign out on its row. That browser is signed out the next time it does anything, and its row disappears.You can't sign out This device from here. Use Log out in the account menu instead.
Click Sign out of all other sessions, under the list. You may be asked to confirm it's you. Every other browser is signed out, and you stay signed in here. The button only appears when another session exists.
Log out everywhere, just below, goes further. It signs out every browser, this one too, and ends your connected apps and access tokens. Use it if you think someone else has used your account. After that:
A session lasts as long as your company allows: 7 days by default, and anywhere from 1 to 30 days. After that you sign in again. Signing out in the account menu ends only the session in that browser.Your sessions also end when:
you change or reset your password (except the browser where you changed it);
your company owner resets your two-factor authentication;