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?
"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 :-)