Explanation from Reddit:

Basically, copypasta is usually blocks of memorable text or quotes that you copy and paste (hence the name) in things like forums and chatrooms, and can almost be seen as 'spam' of sorts, except usually it's associated with memes. Popular examples of copypasta include things like 'I sexually identify as a helicopter', 'a yaoi fangirl and proud', 'this is Bob', the entire script of Bee Movie, and 'Katy the Penguin of Doom'.