On a film set nothing is real - the sword is made of plastic, the marble floor is painted plywood. Foley replaces or enhances that live sound; the result is a sword that rings like metal and floors that echo like marble! During filming, the location sound recordist tries to capture only the dialogue. Microphones are keenly positioned on set to record even an actors slightest whisper without the background noises from camera and crew. Foley helps to add back a controlled background layer of sound to produce a rich and realistic track.
Link to the tutorial.