noun
hand signal; arm signal; flag signal
Primarily used for directing vehicular traffic, but can refer to any manual signaling with hands, arms, or flags.
警察官が交差点で手信号を送っている。
A police officer is giving hand signals at the intersection.
工事現場では手信号で車両を誘導する。
At the construction site, they guide vehicles with hand signals.
信号 is a broad term for signals (traffic lights, signs, etc.), whereas 手信号 narrows it to hand/arm/flag signals.
Compound of 手 (hand) and 信号 (signal), literally 'hand signal'. The reading is the standard on'yomi for each character.