EDF Energy explains The Knock on . . . .
If a player drops the ball forward towards the opposing team’s try line, or loses possession of the ball and it goes forward then this is called a knock on. If the opposition picks up the knock on, the referee gives the opposition advantage, allowing them the chance to do something positive with the ball.
If the opposition is unable to gain ground with the ball, the referee stops play and calls a scrum, with the non-offending team getting the put in.