On this page, you can see a list of custom tables available for the current site. By clicking the Edit () icon next to a listed table, you can view a list of entries in the table, edit them and add new ones. Management of the listed custom tables can't be performed in this section of the user interface. It is available only to site administrators in Site Manager -> Development -> Custom tables.
Detailed information about the Custom tables module can be found in Developer's Guide -> Modules -> Custom tables.