"YTMND, an initialism for "You're The Man Now Dog", is an online community centered around the creation of hosted web pages (known within the community as YTMNDs or sites) featuring a juxtaposition of a single image or a simple slideshow, which may be animated and/or tiled along with optional large zooming text and a looping sound file. Images used in YTMNDs are usually either created or edited by users. Most YTMNDs are meant to expose or reflect the more inane facets of pop culture, and some can be considered inside jokes." - Wikipedia
Basically, it's a website where people upload pictures/animations and put them to a looping song or soundbite. Some are bad, some are good. And some, like this one, are the pinnacle of the internets.