Although at face value could be conceived to be a leet version of the word "true" (which it is in one sense), it is actually more commonly used amongst fans of black
metal, particularly of Scandinavian origin, to compliment a band, person, or thing for it's black
metal or Nordic/Viking values.
It's origins are not certain, although one theory suggests it comes from an old polytheistic
religion of Northern Europe called Ásatrú. In early days, old Norse languages had no word for "faith" or "belief". Instead they used the word "tru" or "tro" meaning "truth" or "loyal to".
So using the word "tr00" in black
metal terms in this case would mean black
metal that is honest, genuine loyal to black
metal ideas and customs.
The word does not necessarily have to be applied to bands or people, in some cases, with the more cultured and perhaps slightly more tongue-in-cheek, might describe a place as tr00 for its Nordic, satanic, spooky or dark vibes.
The term kvlt
may also be used and has a similar effect.