expression, godan verb (-su)
use a turn signal; signal with an indicator
Primarily used for operating a vehicle's direction indicator (turn signal). The phrase is a fixed expression combining 指示器 (indicator) with 出す (to put out/activate).
交差点を曲がる前に、必ず指示器を出してください。
Please be sure to use your turn signal before turning at the intersection.
彼は
He forgot to signal when changing lanes.