A subpage is a page with a title containing a slash (or more than one), such as UserWMWillis/Templates or Barack Obama (1961)/tree. They are used a lot in Familypedia because they make for easy work with templates.
At some time in the past we decided to reduce possible duplication of subpages of individuals' pages by requiring lower-case letters after the slash, e.g. Joe Bloggs/ancestors rather than Joe Bloggs/Ancestors.
See the standard Wikia help box, below, for much more detail.