The area of the field near the 50-yard line. Midfield can be used to specifically refer to the 50-yard line in certain occasions, or can be used as a more generic term to refer to the area of the field outside each red zone. When offenses have the ball at midfield they are said to have good field position and a good chance to get into scoring position. Generally, even if a drive stalls at midfield, a good punter should be able to pin the other team deep in its own zone.
Midfield is a general measuring tool to determine which team is winning the field position battle. If a team tends to have the ball at midfield, it should have more chances to score in a given game. Since field position is crucial to an effective offense, teams will try to pin the other team on the other side of midfield. The half of the field that a team is defending is considered that team's "own territory", while the other side of the midfield is "opposing territory".