It's a copy and pasted block of text usually posted on a message board to troll newer users and as an inside joke between older users. Copypasta can usually be found posted in a discussion about any subject, and will usually be intended to draw out newer users into responding negatively to it, much to the amusement of more veteran users.
Copypasta might have started out as a real post somewhere, but people might have found the story so stupid or amusing that they post the exact thing years later to mess with people. It's much like a group of friends getting together to mock what another person said in the past and laughing about it.
Caption this Meme
