take a knee

Short definition from Wiktionary

Verb

1. 1
   To come to rest, usually briefly, with one of one's knees on the ground.
2. 2
   Of any player with the ball, to kneel and place the ball on the ground, thereby ending the play and indicating they do not intend to advance with the ball.
3. 3
   To cease performing customary activities or duties and kneel on one knee, sometimes as a form of protest.
4. 4
   Used other than figuratively or idiomatically: see take, knee.