A focus session is a timer that pays you for staying off your phone. Open Lock In, set a length, and start. Your blocked apps stay blocked and the clock pays out when it ends.
Extreme Focus is the same thing without a set length. It runs open-ended until you stop it. Turning it on greys out the length row, because there's no length to set.
Use a set length when you know how long you've got. Use Extreme Focus when you're starting something and don't want to be interrupted by a timer ending.
Leaving early. If you end a session before it finishes, you don't get paid for it. The timer running out is the thing that earns, not the timer starting.