...Expand/Contract All
This inserts an expanding navigation menu like the one you can see in the left-hand column.
This type of menu is best for quite complex web-sites, in particular 'documentation' sites like this one.
Usually you will want this sort of menu on all or nearly all pages. It is therefore best to include it on the default page so that you don't have to add it to each individual page. (See Edit Default Page for how to do this.)
See also Expand All button, which is an ajunct to this option.