FULL ANSWER
As the snail moves forward, mucus comes out of a gland located at the front end of the foot, and the snail moves across this gooey substance. The mucus cuts down on the amount of friction that the ground causes for the snail, reducing contact to the point that a snail can crawl along the edge of a knife and not suffer from any cuts. Studies looking into whether the mucus is a requirement for movement are inconclusive, but the mucus clearly helps the movement go more quickly and easily.