A man who lives a life of crime and fucks shit up generally is considered a thug. When someone is considered to be tru thug it means that they are either a) looking and playing the part of a thug. Or b) someone who has left the hood and their thug life for greatness, but still recognises the ideals of a thug.
a) Damn! I'm lookin tru thug!
b) Man, even though you got a record deal, you still Tru Thug.