When you limit or control something you can be said to rein it in. Take care with the spelling. Sometimes people write “reign in” but that’s nonsense.
You can’t reign unless you’re something like a sovereign head of state, and even then you wouldn’t be able to reign “in”. When you rein something in, it’s as if you were figuratively pulling the reins to control a horse or other draft animal.
Likewise, sometimes you hear of a trustworthy individual being given a “free reign”, as if they were being entrusted with unlimited power to rule. Again, that’s incorrect. It’s a free rein: like holding a horse’s reins loose so that the horse can move freely at its own pace.
You can read about the correct use of rein and reign and a whole lot more, in the English Digital Content style guide.