errr super duper simplifies version-
r slur used to be used as a medical term for people with autism/down syndrome and other neurodevelopmental disorders because it basically meant "slow" and implied that those "afflicted" with the disorders were stupid
then it became used in a term to make fun of/harass those with said disorders and eventually was really really bad
eventually it was removed from medical uses but nnow the autistic community has started to reclaim the term as our own because we would rather use it in a friendly way and not an insulting / derogatory way
sorry if thats unreadable