The owner of a page is typically the user that created the page intially.
The user that owns the page receives some special considerations,
and always has permissions to view & edit the page, even if that user would
be excluded from such actions when setting page permissions.
This effectively means that if you mark a page as "administrators" only for example, the
owner of a page would still have access to the page, even if they are not an administrator.
Additionally, the owner of a page always automatically receives notifications whenever
a page is changed.
To change the owner of a page either select the "Change owner action" either from the "Pages" tab
actions dropdown, or from the actions dropdown within a page itself.
The dialog that shows up should be familiar, as it looks and functions pretty much the same
as when you reassign a JIRA Issue. Type the user's name, and select it from the autocomplete
dropdown. Once you're satisfied with the selection, click the "Assign" button, and the page
owner is immediately changed.