Basketball screens, also known as "the pick," are when an offensive player acts as a wall to block or seal off a teammate's defender.
Screens can serve more than one purpose; they can help free up an offensive player when a pesky defender is chasing them around or they can allow the player with the ball to drive to the hoop once his defender is sealed off. Most commonly, picks are used in conjunction with the pick and roll.
Basketball screens are done by planting your feet shoulder width's apart and getting a good strong base so you don't fall over when the defender runs into you. Keep your arms inside so that they do not get in the way of the defender. Using your hands/arms in any way on a screen is illegal. Additionally, it is important that you do not move while in the act of screening. If you move as the defender makes contact, that can be called a moving screen violation.
Check out this video that talks about technique and execution of a basketball screen.