All the pages has a navigation, if you want to add, modified, or delete a navigation item then you can do it from here. Suppose you want to add a navigation item, then you should duplicate an existing navigation item and then change the value of it.
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
Most of the theme also support the dropdown menu. So here is an example of a dropdown menu. You can also modify it like main navigation menu.
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
</div>
</li>