One 11-man team has possession of the football. It is called the offense and it tries to advance the ball down the field-by running with the ball or throwing it - and score points by crossing the goal line and getting into an area called the end zone.
The other team (also with 11 players) is called the defense. It tries to stop the offensive team and make it give up possession of the ball. If the team with the ball does score or is forced to give up possession, the offensive and defensive teams switch roles (the offensive team goes on defense and the defensive team goes on offense). And so on, back and forth, until all four quarters of the game have been played.