The set is usually the second contact that a team makes with the ball. 

The main goal of setting is to put the ball in the air in such a way that it can be driven by a hit.

You can also set when passing if the ball is going over your head.

Advance setters can jump set

As with a set or an overhand pass, the setter/passer must be careful to touch the ball with both hands at the same time. If one hand is noticeably late to touch the ball this could result in a less effective set, as well as the referee calling a 'double hit' and giving the point to the opposing team.