Coast FIRE is when you have enough in your retirement accounts that without any additional contributions, your net worth will grow to support retirement at a traditional retirement age.
Once your net worth has passed the Coast FIRE milestone, you still need to earn enough to cover your monthly cost of living, but you no longer need to save money for retirement. If you save up and invest enough money early enough in life, you can allow your existing investments to compound over time and grow to be enough to retire on at a traditional retirement age. This strategy of "Coasting to FIRE" gives you the freedom to pursue a different job that pays less, shift to working part-time, or just have more spending money to enjoy life.
To learn more about the details and math behind Coast FIRE, checkout my popular Coast FIRE Calculator.