A website can be anywhere from a single page to hundreds of pages. The larger the website, the more important it is to organize it well, and to represent the pages in some kind of a menu system that your visitors will find useful.
For a few pages, a simple, single-level menu works fine. Here all the pages have the same importance, and it is easy to select the one you want.
For a large website, you probably want a manageable number of first-level pages. This is like the highest level of an outline. Within each first-level category, may be second-level pages. There may even be several third-level pages on a very large site.
If you are viewing this page on a desktop screen, or some tablets, you probably see a vertical menu on the left side of the screen. This is a medium sized website, so some of the first-level menu items will have second-level sub-pages below them. To keep the menu manageable, you only see the sub-pages if you are "in" their parent first-level category.