FIRE means "Financial Independence, Retire Early": having enough invested that you can live off it. Enter what you spend, what you have saved, and what you save each year to see your target number and how many years it will take.
All amounts are in today's dollars. Your savings grow at the return after inflation, and the amount you save each year is assumed to rise with prices. Savings rate is what you save divided by what you save plus what you spend. Taxes on withdrawals are not included.