A guard (G) in football is a position that plays on the offensive line of the offense. There are two guard positions on the offensive line, the left guard (LG) and right guard (RG). The guard positions are on either side of the center and before the right and left tackle positions at the end of the offensive line.
The guard's task is to protect the quarterback and running back from being tackled and block the oncoming defenders. Just like every position on the offensive line, the guard is a vital position for the offense's productivity. They are the players that keep the closest defenders away from the quarterback so he has as much time as possible before making a decision on whether to pass or run the ball. Although the guard is an offensive position, he needs to be trained defensively in order for him to be helpful to the rest of the team.
Guards have to be great multitaskers, too. They have to block off defenders (who are also trying to block them) and try to create holes for the quarterback, all while keeping tabs on the field and their teammates positioning. This role can change on the fly depending on the game's circumstance, which is why guards have to be calm under pressure and adjust to how their opponents play.