In mahjong, the turn order goes counterclockwise: 上家 (upstream, left), 对家 (opposite), 下家 (downstream, right). You can only 吃 (chi / sequence claim) from your 上家, so knowing which seat is which matters.
usage
Beyond games, 下家 extends to 'next link in a chain': the next buyer for goods you're reselling, or the next employer when job-hopping.