A Back Screen is when an offensive player comes from behind a defensive player to set a screen at his "back" or from behind where a defensive player's vision would normally be. The term "Back" also comes from the backdoor cut to the basket which is similar to the cut that an offensive player would make after using a Back Screen.
It's important when setting up a back screen that the screener allows adequate space for the player they are screening for to set up an angle to the basket and run their defender into the screen. Ideally the defender should run into the screener, not the other way around. Setting the screen too close can allow the defender to recognize early contact and recover.