The foundation kit gives you the framing for the floor. This is what the shed sits on. Your package contains helpful instructions for a hassle-free assembly. Overall, you'll put it all together, including the foundation kit.
Typically, we recommend pouring a concrete base for the ultimate solution for storage needs. You can even use it as a hobby or activity room. Similarly, you could use plywood if you prefer that. Regardless, you will set up the foundation kit on the material you choose and will then build the shed around that.