Definition
Reps completed after failure with a spotter lifting just enough of the load to keep the bar moving.
An advanced intensity technique: one or two forced reps at the end of a set, on a machine or dumbbell press, with a spotter who knows what they are doing.
The evidence for extra growth over stopping at failure is thin, the fatigue is real, and the joint stress is real. Drop sets and rest-pause give the same extension with less risk.
If you use them, use them rarely and keep them off barbell lifts.