A tree-topper is a special decoration that is placed on the very top of a Christmas tree. Think of it like a crown that a king or queen would wear, but for a tree. It's usually the last thing you put on the tree and it makes it look complete, like the cherry on top of a sundae. Some people use a star or an angel as their tree-topper, while others might choose something different, like a bow or a snowflake. Whatever the tree-topper is, it is meant to be special and make the tree stand out.