For an onside kick to be successful, the kicking team must recover the ball. This is not easy and it's a big risk. If the receiving team recovers the ball they are more likely to score since they're getting the ball close to the 50 yard line.
The kicking team will disguise an onside kick to look like a normal kickoff. The place kicker will make the onside kick difficult to catch, so that it gives players on the kicking team a chance to recover it.