This porn video tag means that the content is related to a homosexual scenario where one or more individuals are involved in a submissive or dominating role. The term "slave" suggests a roleplaying situation where one person takes on the dominant role (master or owner) and the other(s) take on the submissive role of a slave or servant. In this context, "gay" refers to same-sex relationships between men. So overall, the tag describes a homosexual scenario featuring male characters with roles that emphasize dominance and submission.