This article refers to the actual riddle, not the meme video called Riddle of the Rocks. That article is here.
The answer to the riddle is L E M O N. Only the Ancients and a select few others know the answer, and those who find a way to learn the answer must perish for accessing the knowledge of the Ancients.
In the video, the riddle is a sentence being said in reverse. if you put that scene backwards, it says "To be fair, you have to have a high IQ to understand Rick and Morty." The video revealing that can be seen here. (WARNING! SPOILER IF YOU HAVE NOT SEEN RIDDLE OF THE ROCKS)