A soft block is one in which the blocker's intent is to slow the attack down, not to stop it. The idea is to put your hands up in front of the hitter in such a way as to pop the ball up and back into your court.
The best soft blocks slow even the hardest swings down and send the up into the air in an arc that comes down to your team almost as easily as a free ball.
Because of the soft blocker's presence of mind, what could have been a kill for the other team becomes a scoring opportunity for your team. If your team can get an easy pass, a good set and a solid swing, your soft block just created a point-scoring opportunity that your team would otherwise not have enjoyed.