Basically, it's a little like Yelp for races (but much less douchey). You look up a race you've run (it's a pretty good database so far & growing, and you can always request to add a race if it's not currently in there), rate it on a few different aspects (difficulty, production, scenery, etc.), and write a little review for it. (You can also add links to more detailed race reports or images if you want.) There are also options to include your time or rate your performance for a race if you want. All the races you've rated show up in your private "Staging Area" (dashboard/home page).
If you search for a race, you'll see all the reviews people have written along with its average rating, a map of its location, and historical weather data for recent years (<--BRILLIANT).
Now, this next part, I really should not tell you. But I am a good person, so I will. Until January 22nd, they are running a sweet contest with prizes that include an early bird entry to a Brazen Racing event, shoes, a RoadRunner Sports gift certificate, etc. All you have to do is join & review your races (at least one of which I think has to be a Brazen event). (Contest details here.) I still have a huge head start on most of you, so better get typing!
Also, ICYMI, I am running a race (!) tomorrow, so wish me luck. :)