A slot is a placeholder that users can fill with their own markup. The slot may exist outside a Web Component or inside, in conjunction with a template or a Shadow DOM (or both).
Slots Explained
Many types of components, such as tabs, menus, image galleries, and so on, need dynamic content to render properly. Just like a browser’s built-in element <select> expects <option> items, a <custom-tabs> may expect the actual tab content to be passed. And a <custom-menu> may expect menu items.
The code that makes use of <custom-menu> ...
