Sub-Menus are pretty much what they sound like. Instead of having all your pages on one large menu, you can have lower levels of navigation that allow users to move through your site in organized areas. Sub-Menus on a mobile website can be equated to a drop down on the main menu of a standard website.
Utilizing sub-menus on your mobile site can improve navigation and increase the time people spend on your site. Sub-Menus can be used strategically in a few specific areas. Here are some tips for utilizing sub-menus on your mobile website.
Restaurant Menus and Product Catalogs
This is a great place to use sub-menus. Instead of plopping your entire catalog of items or your entire restaurant menu on one large page, you can use sub-menus to break it up and organize. For instance, a restaurant could make a sub menu with 3 pages: "Breakfast", "Lunch", and "Dinner" - each of which are their own distinct page outlining the food items for that meal. The sub-menu in this situation divides the content in an organized way, so you do not have too much on one page.
A retail store with a plethora of items could implement this same strategy to break up their catalog into departments or types of products. The degree of specificity you take it to is up to you. You could have 4 levels deep of sub-menus to clearly organize everything, or keep it simple with a 2 level structure.
Photo Galleries and Media
You can also organize media on your mobile website with sub-menus. The simplest way to break this up is by media type. For instance on your main menu you have "Media" which opens a sub-menu. This sub-menu then has "Videos", "Photos" and 'Audio Clips". Each of these pages feature media of that certain type.
You can also break your media up more categorically. Let's say you are a non-profit organization who does a variety of activities. You could have a sub-menu called "Media" and a photo gallery on that sub-menu called "Building Projects". In this gallery you feature images of various building projects. You could have more galleries, all featuring images of their own distinct activity type.
Mobile Website Sub-Menus
No two organizations are the same. No two mobile websites are exactly the same. It is important to figure out what works best for your mobile site. Sub-Menus can be very useful, but be sure to not overcomplicate your mobile website or make your navigation so complicated that users get lost.