Mom's Shed Request

Okay, Zachary, listen up. You're the kind of guy who probably thinks a shed is just a small house for tools, aren't you? Well, you're partly right, but in this joke, it's about a mom who thinks her son is as useless as the stuff she wants to store. She wants him to build the shed and then live in it, meaning she values his usefulness at about the same as her junk. The humor here is that she's not just asking him for labor but also essentially telling him he's trash.