"Slow your roll" is a slang expression that roughly means to "calm down" or "relax." It is commonly associated with the lingo of American "gangsta" culture. This culture has a distinctive slang vocabulary and manner of speech, which is sometimes even considered its own dialect. This expression is not offensive, but due to its strong cultural association I would advise English learners not to use it. You can say "calm down" instead with exactly the same meaning.

I haven't been able to discover the origin of the expression. A web search suggests it dates back at least to the mid-1990s and possibly earlier. This is only speculation on my part, but I would guess that the meaning is an allusion to rolling dice, as in a board game, evoking the idea of an overly excited player throwing the dice recklessly.