In this porn video tag, "ftm" stands for "female to male." The phrase describes a scene where a person with female anatomy is engaging in sexual activity with a male partner or a transgender male (FTM) individual. The term "riding" refers to the act of sitting on top of a penis or a dildo while moving up and down, a common sexual position known as "cowgirl" or "reverse cowgirl." Finally, "dildo" is a sex toy designed for penetrative play, often used in situations where an actual penis is not present or cannot be used. In summary, the tag describes a scene where a person with female anatomy is using a dildo to engage in penetrative sexual activity (riding) with a male partner or a transgender male individual.