I was reading a book and saw a character say "C'est quoi ce brol ?" I couldn't find the word in any dictionary. I do wish there was a decent Urban Dictionary for French, but there isn't. What does this word mean and why isn't it in dictionaries?
It's an informal expression presumably confined to use in Belgium, which accounts for the difficulty in looking up the word.
Doesn't "C'est quoi ce brol ?" remind you of a similarly structured "C'est quoi ce truc ?"? Yes, "brol" basically means "truc", though "brol" has a more pejorative sense like "what is this stupid thing?".
On the other hand, "brol" can also mean "a place in a shambles", but the first meaning applies to its use in "C'est quoi ce brol ?".
"Brol" is a belgicism. The general meaning is some kind of messy situation. See here: https://mobile-dictionary.reverso.net/francais-definition/brol
For example, entering the room of your kids, you could probably more than often ask:
C'est quoi ce brol ?
This is probably the meaning in your example
I've heard it quite often used also to describe an unidentified thing. Kinda as a synonym for "truc" or "machin". You could then ask the same question about let's say a mysterious email you received or a strange object on a colleague's desk, or some piece of contemporary art...
All my examples are real life ones, I use them on a regular basis.
Note this would only be understood by Belgian French speakers and is very familiar, while being slightly pejorative, so you may not be able to use it a lot yourself :-)