"Tgirl" stands for "Transgender Girl." This term is used in the context of adult content to describe individuals who identify as trans women. "Cumshot" refers to the act or moment when a man (or, less commonly, a woman) ejaculates and their semen comes into contact with another person, an object, or themselves.
In the context of this porn video tag, "tgirl-cumshot" means that the content is likely featuring a transgender girl who engages in sexual activities leading to her partner's ejaculation and the tgirl receiving or interacting with the cumshot.