The weak side in football is the side of the field opposite the side that the tight end (TE), a player on the offense, stands on. The weak side is used in by defensive coordinators when designing plays. The free safety, a player position on defense, stands on the weak side of the field. The weak side has fewer people, which is one of the reasons it's considered the weak side. The side of the field with the tight end is called the strong side.
The weak side linebacker needs to be the quickest out of the strong side and middle linebacker. His main role is usually to stop the run. He often steps in to help with pass coverage. He needs to be quick on his when faced with jam ups and traffic on the field.
The weak side linebacker can't shy away from making a tackle. He should be a player that can read the field in an instant, see openings and step into place to help out the other defensive members. He should be familiar with a one-on-one scenario, as they might happen if the offense sees an open lane on the weak side of the field. Don't be fooled by the name, the weak side in football is where a lot of the action can occur.