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.
Coach: John, listen - we got 5 seconds remaining, pass the ball to Porter, Porter - you'll take a knee and end this game.