It means that the value proposition at face value seems preposterous to most people, e.g "watch some guy walking around the entire day" (justin.tv) or "watch some random guy play a video game" (twitch). Where someone thinks later, "Who the hell would have thought that would have been a business".

It's also sometimes used for something someone did for "shits and giggles" or as a side gig instead of spending more time on their serious day job or "primary project" (i.e. all those internal tools that grew into products).