(also abbreviated n.s.f.w): exactly what it means. Not suitable for whites (ie unfit for white people consumption or for consumption by white culture).
If you watched 8 Mile the movie, the one with Eminem, you know what not suitable for whites means.
(Also abbreviated n.s.f.w): means exactly what it's supposed to mean, ie practices from non white cultures that are unfit for cultural consumption by white people.
Even if Eminem is white some of his music sounds like black rap so it may be not suitable for whites.