In football, strong side is a term used to describe a side of the field relating to the alignment of the offense. When the offense has a tight end in their formation, the side of the offensive line the tight end lines up on is called the strong side, while the other side of the field is called the weak side. It is called the strong side because that side of the field has an extra blocker. Most runs in football are toward the strong side of the formation.
The strong side linebacker typically focuses on tackling and is the strongest tackler of the three. Depending on the play, he provides pass support or plays the more traditional linebacker role. The strong side linebacker will also fill in gaps according to each play, which is why it's important to have linebackers that can read the field at a glance and decide where he is most needed.