FIRE stands for Financial Independence, Retire Early: accumulating enough invested money that its returns can cover your living costs without a salary.
The target is calculated from annual spending rather than income — commonly by dividing yearly expenses by a safe withdrawal rate, with 4% being the familiar shorthand, roughly 25 times annual spending. That formulation explains why spending is the dominant lever: reducing it lowers the target and raises what you can invest at the same time. In practice most people who pursue it aim at optionality rather than never working again.