hey guys,i'm new in ultimate guitar.
i would like to know what's the hammer on; i know it's used in the slap,but...how?

thank's for yours answers.
When you play a note, say on the 7th fret G string, and then without picking again, put your other finger down on a fret above it, say 9th fret G string. If you're anything when it comes to theory, its the guitar player's version of a slur.

EDIT: If you're a bassist, you're looking at "Popping" or "Fingering?" the string instead of picking. Popping and Slapping go hand in hand I think.
( To make cleaer what bartdevil_metal said )

For instance:

You would pick the 7th Fret and then place your finger on the 8th fret without picking.