They were cute little flash animations that people traded around even before YouTube was a thing. This was a dial-up modem quality of humor, so it was going to be low res and short and with minimalist sound/animation design. Sometimes you'd get a full one-man animated music video done in paint and midi or whole comedy riff done in pixel art. Other times this was some ytmnd.com (you're the man now, dog) tier content, which was literally just clips from movies/songs/interviews over eyeball gouging stills.

They were churned out in bulk by lots of people who were just throwing stuff against the wall to see what stuck. Truly the avant-garde of online media, with all that this entails.

At the time it was interesting, because so much of it was new and original. Now its practically cliche. Jay Leno posting "All Your Base Are Belong To Us" was the death rattle of the trend. But for a little while, it was genuinely a lot of fun - like drinking water out of a firehose.