Baby-sitter Porn Videos
"Baby-sitter" in the context of a porn video is a roleplay or scenario based on a woman being hired as a babysitter for a family. This often involves the babysitter being seduced by one or both of the parents, or being caught by them engaging in sexual activities with someone else. The term could also be used to describe a scene where a young woman is depicted as being babysat by an older character, usually in a more domineering role.
In summary, it's a scenario-based tag that refers to a situation where the female character takes on the role of a baby-sitter or someone being cared for like a baby-sitter would.