Okay, so you know how you have your favorite books and movies that you love and talk about all the time? Middle-earth canon is kind of like that, but it's about the stories and information that come from a specific place called Middle-earth.
Middle-earth is a made-up realm where characters like Frodo and Bilbo Baggins, Gandalf, and Legolas live. This realm was created by a man named J.R.R. Tolkien who wrote many books about it. Those books are kind of like a big family of stories, and all of them together make up the Middle-earth canon.
When we talk about the "canon," we mean all of the stories and information that come from the original creator, in this case, J.R.R. Tolkien. Think of it like a big rulebook that tells us what actually happened in this made-up world.
So, if someone says that something is "canon" in Middle-earth, it means that it comes from the stories and information that J.R.R. Tolkien made up. Anything that doesn't come from those books isn't technically part of the Middle-earth canon.