Cart is a Model
Cart.objects is a Manager
cart = Cart.objects.get(id=1)
cart is an instance of a Cart
cart.items is a RelatedManager which can return the items related to the cart.
If you haven’t done so already, go through and play around with the examples in the Django tutorial and then the Django rest framework tutorial. From that, you should start to understand how relations work in the ORM, and then how Django rest framework magic can turn orm models into rest api endpoints.