A pre-oiler is a unit that adds oil into the oncoming air stream. You can get one for under 50$ at harbor freight.

You need to have the oiler on the intake tract of the compressor. The normal refrigerant circulated in an A/C system has oil in it to lube the pump so that the system can be 100% sealed. If you are using a newer pump you will need to be sure that you get a oil to it or it will burn up.