A component is a reusable piece of a website, such as a button, menu, or card. It is like a building block that can be used in several rooms while keeping the same basic shape.
If every page uses the same button component, changing that button once can keep the whole site consistent. A component can also accept different words or pictures for different uses.
Reusable pieces make large sites easier to maintain, but they should stay understandable. A component is simply a useful part that can be used again, not a special kind of page by itself.