The header menu appears on every page of your online store. The header menu is usually displayed as items across the width of the header, or else as a list of items in a sidebar. A customer will probably look at your main menu to find your products and information about your business, like an "About us" page.
You can add a drop-down menu from any of the menu items in the header menu. A drop-down menu is a sub-menu from the header menu, and is usually displayed as a list of items connected to the header menu item. Drop-down menus are a good way of organizing similar items: groups of webpages, blog posts, store policies, products, or collections. For example, if you have a lot of products, then you might want to add them to collections and then use a drop-down menu from the main menu to organize the collections. This can help a customer find the type of product that they're looking for.