Reee is the war cry of a battle hardened memer, and 4chan user, who uses the glorious scream to scare every normie away from the page.
It is similar to an autistic screeching, though it is said to represent the agitated croaks of frogs. The “r” is said to stand for the word repost, to warn users if a meme has been stolen from its plain of creation and uploaded somewhere else.
Shortly after the first post, 4chan got filled with posts, featuring Pepe the Frog and people’s voice clips of screeching. After that, a Redditor finally asked the question, why people are doing these screeches, to which people just responded that 4chan users are autistic.
In 2017 a game similar to Flappy Bird got made, featuring a screaming Pepe, who is collecting tendies, though Apple banned the app from its store, calling the frog “objectionable content”.