Vinny demoing some snow footsteps with a starch bag.
From Wikipedia: Foley is the reproduction of everyday sound effects which are added in post production to enhance the quality of audio for films, television, video, video games, and radio. These reproduced sounds can be anything from the swishing of clothing and footsteps to squeaky doors and breaking glass. The best foley art is so well integrated into a film that it goes unnoticed by the audience. It helps to create a sense of reality within a scene.
Aran Tanchum, our good friend and longtime foley editor (for approximately the second half of Avatar, and all of Korra), has started a Tumblr blog about the art and process of foley. On the other side of the mic is Vinny Quisetti, the “foley walker,” the person performing the sounds to match the movement in the footage. Unfortunately, Aran can’t show clips from the series, but if you are interested in filmmaking, this is a good look into some techniques and tricks of the trade.