The side of an offensive formation that has more players at the line of scrimmage. This usually refers to the side that the tight end is lined up on, but it can refer to a number of different alignments. The easiest way to denote the strong side is to examine which side of the formation has more players lined up on the offensive line. The strong side is considered a better side to run to, since the offense has more blockers on that side to open holes for the ball carrier.
The strong side is the primary side for running plays. A tight end and fullback can give the offense two more blockers to take on the defense and create running lanes for the tailback. The rules that dictate where offensive players line up force an offense to have a certain number of players to be directly on the line of scrimmage, and this usually creates a strong side/weak side difference. The strong side can change before the snap depending on which players go in motion, as a tight end can switch from one side to the other. Defenses will respond to formations by rotating players toward the strong side, usually a linebacker or strong safety.