Stuff is "everything that is not food, or weapons". Like a chain, a computer chip, batteries, socks, first aid kit, thread, a piece of leather, etc.
Say you have one unit of stuff. And you're really looking for a first aid kit. Well, scratch that unit, and you have a first aid kit.
If you want to barricade a door, you use one unit of stuff (and not needing to list out the planks, nails, hammers, screws, screwdriver, battery for the screwdriver, the correct bit, etc). It just simplifies the listing of items the group will pick up as the game goes along.
Instead of stating in detail every single item that is gathered, it's just... units of stuff.