Subpages allow you to add extra pages to a Moodle module.
Subpages should not be confused with another popular resource type in Moodle, the regular 'page'. If you add a 'page' in your module, it is simply an extra plain html page lacking the option to add resources and activities. These options do appear on a subpage, which is very useful for content-heavy modules, or cases where you want to make a particular collection of resources available to a specific group (for this you may want to use Groupings).
You have the same full range of options to add resources or activities, along with text, links, or html (embed code) on a subpage that you have on the main page of the module.
If you do make use of subpages please read the important caveats below as there are some bugs and issues to be aware of.
1. The new 'activity chooser' does not work on subpages - in order to add a resource or activity you have to revert to the old menus (click here to find out how to do this).
2. The Subpage plugin has a bug - only sections that have a resource or an activity in them are displayed when editing is off. So if you have sections that are just labels (text / links only), these are hidden from students as Moodle doesn't regard these as containing anything.
A workaround - you can always add something and then hide it in order to make an invisible section visible.
3. Any content that sits within a Subpage will confusingly also be shown (when you have 'editing on' only) at the bottom of the main page, under a heading Orphaned Activites. This is a bit odd but just seems to be the way Moodle works - it can't tell the difference between left over content that falls outside of the topics on the main page, and content which has been deliberately moved to a Subpage. Just don't delete anything from Orphaned Activities as it will go from the Subpage as well.