Foley is the process of adding sound to picture in the studio, specifically human made sounds such as footsteps, clothing rustle and the rattle of objects which are best performed live to picture. 

If any of these sounds occur during dialogue, effort will be made on set to silence or mute them so that they can be created later.  In addition to permitting much more flexibility during the post production process (see below) this is also necessary because the dialogue tracks may be lost for foreign language mixes.

It is important to note that a foley artist will often choose sounds which convey the emotional or psychological aspect of the object or action.  Thus the actual object that the screen actor is using may not be used by the foley artist and thus will not be the source of the sound the audience hears.  For example, if a character is handling a rifle then a ratchet-type tire jack may convey (through its weight and rattle) more of a convincing menace than a real bolt action weapon.

The person who does this may be known as a Foley Artist or Foley Walker as much of the work involves replacing or adding footsteps.