When young children in America play football, it is usually a version of the game called Flag Football. In this version, the players are not permitted to tackle or push the other players. In a more advanced version of football, including professional football, players are allowed to grab another player who has the football and do their best to stop them from scoring a touchdown. This results in them tackling the player down to the ground so they cannot run with the football. While tackling is allowed and useful, there are rules to make sure that players do not use unnecessary force against their opponent. 

Once a player is already on the ground, there is no reason for another player to jump on top of them. This would be an example of unnecessary roughness. In the story of Junga the Dancing Yeti in Yeti, Set, Go!, Groth was already tackled by the Boar twins, so there was no need for Brenda to jump on top of Groth as well. 

When a player is too aggressive and shows poor sportsmanship like this, it can really be bad for their team. The Umpire (or Coach during practice games) is the person who decides if someone receives an unnecessary roughness penalty. If this penalty is given to a player, their team either loses 15 yards on the field or the competing team will gain 15 yards to help them.