A double team in football is a tactic which involves using two players on your team to cover a single player on the opposing team. The strategy is usually utilized when there is one particularly skilled player on the opposing team who can not be covered by just one player.
Double teams are used in three different situations:
Pass protection, typically the job of the offensive linemen, means protecting the quarterback so that he can pass the football. When a team double teams in pass protection, that means the offense is putting two players on a skilled pass rusher to provide extra protection to their quarterback.
Double teaming on pass protection is often an adjustment made when a pass rusher is consistently breaking through the offensive line. Or, if a team has a star pass rusher then the other team may double team them from the start of the game.
The success of a running back is heavily dependent on the offensive team's blocking. Without good blocking, there will not be any holes for the running back to burst through. If a team needs to open up a hole on one side, they can double up on blockers to try to power through the defensive line. This strategy can be implemented on just a couple plays throughout the game, or more regularly to target a talented defensive lineman.
If there is a very talented receiver, the other team's defense may want to put two defenders on him to make it harder for him to get open.