what’s the meaning of the field wp_capabilities in table wp_usermeta

The wp_capabilities saves the value as serilized array, you can try it in your php or for this example here: http://blog.tanist.co.uk/files/unserialize/.

The Code:

a:1:{s:13:"administrator";b:1;}

Is:

Array
(
   [administrator] => 1
)

Meaning the user is an administrator.

You can add new roles to the database by running the function add_role, and only run it once!