The glue, or adhesive, traps the mouse and prevents it from leaving.
The greatest problem with this kind of trap is that it does not kill the mouse-- unless you leave it enough time for the mouse to starve to death (a cruel death indeed).
Live release is often difficult without seriously hurting the mouse.
How to Bait Glue Traps.
Bait is normally placed on of the adhesive, however you'll need to put it in the middle of the board for this to be reliable.