"BBC" stands for "Big Black Cock," referring to the size and color of the male genitalia. In this context, "trainer" implies that the individual possessing the BBC is either preparing or teaching someone else how to use it in sexual activities, such as penetrative sex. The tag suggests a video where a large, black penis is being showcased or used for training purposes. It typically appeals to viewers who have a specific preference or fetish for larger penises.