A ruck is when one person gets tackled and goes on the floor and keeps the ball the furthest away from the opposing team. If your team keeps it close to the other team then the other team will take the ball and continue the game.
In a ruck there is also something called a “desputa”. A desputa is when the opposing team can take the ball out of the ruck. To avoid this you can clean the people that or trying to take the ball away, when I say clean it means take the people out and continue the game by taking the ball away from your team and passing the ball