A hotdurger (a portmanteau of hotdog and hamburger) is a sandwich consisting of a frankfurter sausage (hotdog) served on a split kaiser roll (hamburger-bun).
In Reggie's Big (Beach) Break, the Shore Shack during an extremely busy period runs out of hamburger meat and hotdog-buns, but (for a while) had plenty of hotdogs and hamburger buns available. So Tito starts serving hotdurgers.