It's an aggregate site. So essentially critics from newspapers around the world and websites either submit reviews or are contacted to submit them. Not sure how that part works. But anyway, the top critics are indicated as well and those are normally the ones from publications with a high circulation rate like USA Today or the Washington post or the New York Times, etc. When they submit a review they also tell them whether it's rotten or fresh. So the score is based on how many critics give it a fresh rating. So if a flick has 120 reviews, the score is a percentage of that 120 who recommend it.
Now of course just because a critic gives a movie a fresh rating doesn't mean they think it's perfect. The problem with the site is the binary nature of "fresh" or "rotten." There's no in between and I've heard critics say that frustrates them.
It's not a joke site tho. It's reputable critics along with some you've probably never heard of either but it doesn't mean they're less important. It's an aggregate; it gets all the opinions together which in theory makes it easier for you to get a full scope of the opinions out there on a flick. It's not end all be all tho and you can believe or trust whomever you want to believe or trust
