"Son comes inside mom" refers to a porn video where the son penetrates his mother, and at the point of climax, ejaculates (comes) inside her vagina. This tag is specifically describing a scene that involves incest or family role-play. It is intended for adult viewers who are interested in this specific type of content. Note that this act could potentially lead to unintended pregnancies in real life, but within the context of fantasy and adult entertainment, it is a common theme in pornography.