When a defending player, with his back to the ball, deliberately interupts a receivers line of vision with an incoming pass. The defender often waves their arms and hands above their shoulders to try block the receiver's view.
Faceguarding does not involve physical contact with the receiver, nor does the defending player make an attempt at playing the ball.
In NCAA football faceguarding is a penalty only if contact is made, in which case it's pass interference. Meanwhile in the NFL it is a legal practice ( provided there is very minimal contact).