It stands for “reddit,” which was actually the official name they used for subreddits. They were actually supposed to be just called “reddits” (so like, “come visit the Gaming reddit!”), but that’s stupid so nobody called them that. The “/r/” is a vestige of that.
Quick Google search:
We actually called them “reddits” for a while. So you’d read your reddit on reddit, and maybe you’d reddit on your reddit on Reddit and… well that’s a lot of reddit-ing (the r/ is a vestige of this time as well). But that got pretty confusing, so we moved to “subreddit” (like sub-domain, but… reddit).