The Jackpot is a sexual term for having your penis at the top of a girls vagina. Quickly pulling the penis down the vagina saying "DING DING DING JACKPOT!!!" then furiously jamming your penis in her vagina. (This might hurt a lot if she is tight.)
Person 1. I was banging that girl and pulled a The Jackpot!
Person 2. DUDE! DID IT HURT HER?