The overall term for a defense's efforts to stop the opposing team from completing a forward pass. There are two different primary types of coverage: man-to-man and zone. Most coverages use a blend of the two in order to keep as many options open for a defense as possible. Any defensive player can be involved in coverage, but it is the primary responsibility of the defensive backs, with the linebackers playing a secondary role.
A team that is effective in coverage is more likely to keep the opposing team from being effective in the passing game. A player in coverage is responsible for keeping an eligible receiver from getting open enough to receive a pass from the quarterback. The cornerbacks are the most important individuals in coverage, because they are most likely to be matched up on a single player for the duration of the play. Teams will use a variety of formations and alignments to maximize their success in coverage.