This tag refers to a specific type of porn content. "BBC" stands for "Big Black Cock," which is a slang term used to describe a large black penis. The word "White" in this context indicates that the scene involves a white person receiving oral sex or penetration from this big, black penis. So, the tag "bbc-on-white" describes a porn video where a black performer with a larger than average penis is engaging in sexual acts with a white partner.