Explain what an item is?

Love the book so far, it's a great intro. On https://danielkeep.github.io/tlborm/book/mbe-syn-macros-in-the-ast.html, you talk about Items and `impl` Items, but it's unclear at this point what an Item is. We refer to them again in https://danielkeep.github.io/tlborm/book/mbe-syn-expansion.html. Could you add a brief explanation of what an Item is in the first link there?