Foley is the art of creating practical sound effects for movies, TV series, and even video games. It is sometimes called "the invisible art" because when well done you don't even notice it nor pay attention to it. However, most notably, if it is not present, this absence becomes very conspicuous.
Most people actually have never even thought how exactly the myriads of small sound effects in movies are created, and aren't aware of the techniques. Most people, if asked, would probably just assume that all the sounds happening in a scene of a movie were simply captured live by the filming crew microphones as the scene was being performed. And this is part of the illusion of foley: Making it feel like it is exactly what's happening on screen. Making it not sound like it has been added in post-processing.
Of course these sounds can't be simply recorded directly from the scene being performed. Most of these sounds are too inaudible to be captured (and would get swamped by louder noises). Some sounds may be way too quiet, others way too loud. Many times the sounds in the live performance are not proper (for example because the prop materials being used aren't what they look like; for example an object that's supposed to be metallic actually isn't.) Oftentimes desired sounds simply don't happen in the live performance in the first place.
When this is pointed out, many people would then probably think that the sounds are added in post-processing using computers and vast libraries of ready-made sounds. Of course foley predates computers and all forms of electronics by quite a lot, and to this day trying to create all possible sounds happening in a movie using computers and libraries of pre-recorded sound samples is completely infeasible. There are only so many sounds that can be put into a library of sound samples, and it would be an enormous amount of work to try to mix them into a movie, making sure that they sound proper, and that there are no obvious repetitions. A movie may require literally tens of thousands of tiny sound samples, all properly timed, and all of the proper type, depending on the context.
Even after a century of film-making, foley is still the best and most practical way of creating sound effects for movies. Foley artists literally create sounds using all kinds of physical objects, in sync with each scene of a movie. (Most typically they will be watching the scene on a large screen, and creating sounds using all kinds of objects as they are watching, in sync with what's happening.)
This kind of "live" foley allows for a virtually endless amount of creativity and variation, to produce that perfect sound for a particular thing happening on screen.
The foley artist will take into account everything that's happening, no matter how small of a detail, and consider if it warrants creating a sound for it. Many of them are things that normal people wouldn't even notice or think that it should make a sound. But it's all these tiny sounds that make the movie feel so real.
To an increasing manner foley is also used in video games. When foley is missing, or done poorly, its absence makes the scene feel very awkward and unrealistic (especially in cutscenes).
The next time you are watching a movie (perhaps for a second time), try paying attention to all the sounds that can be heard even in very mundane scenes where nothing particularly special is happening.
