An American football term for when the quarterback drops to one knee immediately after receiving the snap, thus automatically ending the play.
The use of taking a knee comes only near the end of a game when your team is in possession of the ball and holds a narrow lead (by a touchdown or less). If you execute a run or pass play, you risk turning the ball over and allowing the opposing team to gain possession and a chance to score. By taking a knee, you are able to run down the clock without putting the ball at risk.
A boring, if strategically important, play.
Take a knee is also used as a general term in the sports world meaning both to take a rest and as a request by the coach for eveyone to sit down and listen.