Essentially, their intent is to protect the interests of the "old" Afrikaner nation. I think that their fear is that because Africa is a continent with a huge black majority that their identity as a people is at risk... which I guess is a fair concern.
Whether or not you and I subscribe to the ideals of the HNP isn't really relevant to me - what I find it interesting that a people feel the need to protect their identity with such fervour. It's not really something that I'm able to identify with.