REEEEEEE" is an onomatopoeic expression of intense rage or frustration typically associated with the Angry Pepe character and used by those who frequent the /r9k/ board on 4chan. The expression is often associated with the Autistic Screeching meme, however it is intended to represent the unique croak produced by several species of frogs when agitated.
https://media.tenor.com/kmObZVUZkNwAAAAC/kylo-ren-8pack.gif