I have trouble understanding as well but I usually like the tune rather than the lyrics. The only rap song where I heard and understood the lyrics was Stan by Eminem. Very sad song.
I think Eminem is amazing, and I watched Jay Zee live at a festival a couple of years ago and I was utterly mesmerised with his skill - rhyming seamlessly for over an hour with pause.
But by and large, as a sixty-six-year-old white man, I accept that I am somewhat wide of their demographic!!