The double attack is the tactical foundation behind most won material. Because a player only gets one move to respond, attacking two targets simultaneously forces a concession somewhere. The queen, with its enormous range, is the most frequent double-attacker, but every piece — even a humble pawn — can deliver one.
Many combinations are simply a forcing sequence that ends in a double attack: a check or capture that drags an enemy piece to a vulnerable square, followed by the blow that hits two things at once. Training to spot loose, undefended pieces is the fastest way to start finding double attacks in your own games.