Coast FIRE is the point where you have saved enough for retirement that you no longer need to contribute another dollar. Compound growth is expected to carry you the rest of the way.
The trick is to invest early and consistently. Once you reach Coast FIRE, you can shift your focus to covering current expenses without worrying about adding to retirement accounts.
It is a flexible milestone. You can use it to take a lower-paying job, work part time, or simply enjoy more financial breathing room.