A basketball defense fundamental in which a defensive player who is two passes away from the ball adjusts his defense to help another teammate defend the ball handler.
To understand help-side defense, first you have to understand what the help-side is. The help-side, also known as the weak-side of the court, is the area of the court away from the ball (and therefore the ball handler). In man-to-man defense, a defensive player on the help-side is typically already guarding another player (and in zone, guarding a specific area). In order for that defensive player to provide help-side defense, he must maintain his current defense while also helping to prevent a drive to the basket by the ball handler.
The best practice for help-side defense is for the helping defender to move away from his own man or defensive assignment towards the center of the court, but only enough to deny a clear lane to the basket, while also close enough to return to his original defensive assignment. Most coaches will teach defenders to drop about a step below the line of the ball and open up their body, so that when they are facing away from the basket, they can see both the player they are guarding and the ball in their peripheral vision. By not being too far away from their assignment and being closer to the defender they are helping, they have enough time to react and defend either a pass or a drive to the weak side.