Flash Exposure Lock
A control that fires the metering preflash in advance and holds the resulting power setting for subsequent frames.
In depth
Locking lets the photographer meter the flash off the subject and then recompose, so a bright or dark background cannot skew the result. It also gives consistency across a sequence, since TTL would otherwise re-measure and possibly re-decide on every frame.
Examples in practice
Metering flash on a bride's face at the centre of frame, locking, then recomposing so she stands to one side of a dark doorway.