I will just quote these two posts, though I believe there was a better explanation somewhere:

>>61838
>Meaning of the term "shitposting" is unclear, so don't use it. It could either be used to refer posts everyone agrees to be shitty (which will probably break R8) or to having fun making very high-quality posts
>>64890
>it was later decided that the term "shitposting" itself was Rule 8-esque, had too many wildly varying interpretations (from "making low quality posts" to "any form of humor/joke on the internet"), and a lot of our non-imageboard-derived users had no idea what it meant