What does SH in baseball mean?

SH in baseball stands for "Sacrifice Hit" - also known as a Sacrifice Bunt.

It is when a batter comes up with a runner or two on base and less than two outs, and intentionally bunts the ball in a way that the fielders have to get him out- which allows the other runners to move up a base or even score. The batter "sacrifices" himself to allow the other runners to advance.