"Fill" in the context of a porn video tag typically means a sexual act where one person, usually a male, penetrates another person's orifice - be it their mouth (oral), anus (anal) or vagina (vaginal). This could also imply ejaculation, when semen is expelled from the penis, often into a person's body cavity or on to a surface. It can also be used in a non-sexual context to denote the act of completely filling something up, such as a mouth with food or drink. However, it's most commonly used in explicit sexual content.