The term pop your cherry is slang for the breaking of the hymen. The hymen is about a half-inch inside the vagina and can be broken with penetration someone and popped their cherry, it means the person they had sex with was a virgin.
When a girl who is a virgin has sex for the first time with a guy. A piece of skin inside of her vagina is moved once the guy has "put it in". Making her bleed. When your cherry is popped you only bleed the first time.
When a female virgin first has vaginal intercourse, or some object is inserted into the vagina, causing the hymen to rupture, resulting in bleeding. The term