A Blindside Screen is a type of screen that happens behind a defensive player's back, where they can't see it coming. The player setting the screen positions themselves on the defender's "Blind Side," which is why it's called a blindside screen.
The screener stands directly behind the defender, facing away from the basket.
They leave about one step of space between themselves and the defender's back. When the offensive player with the ball moves, the defender tries to follow them but runs into the screener they didn't see.
This unexpected screen can free up the offensive player to take a shot, drive to the basket, or pass to an open teammate. Blindside screens are effective because they catch the defender off guard.