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.

ADJ: see true. can be used to agree with any statement, or to express a common interest.

For True
internet slangg

Tr00 as in for real, blacker than black, eviler than evil,
Kvlt as above,
Metal, heavy metal music, chiefly Black Metal.

To sum it up, Black Metal that is not phony.