I've just researched the term, having heard or read it several times without really getting it.
Halfway through the Wiki explanation, I still didn't get it.
What the hell is so special and hilarious about digital pictures with subtitles? This stuff has been going on for the past decade; we did similar stuff back in '96-'97 at Uni. Yet, the web pop culture seems so (im)mature now that it only takes one funny caption, allowing for replication and customization - and then everyone goes ape shit, invents new terms, entire fandom websites, not to mention taking up space in otherwise respectable news media.
You've got to hand it to American pop culture - they really know how to arouse each other when they're bored. That's probably how cheerleading and professional wrestling was invented.