If YES, it means that this role is granted to user as a default role.
If NO, it means that this role is granted to user, but not as a default role.


I guess the above explanation does not help much, does it? So the right question would be, what is a default role and what is nondefault role.

A default reole is a role that is enabled automaticaly after you log in. On the other hand, a non-default role is a role that has been granted to you, but will not be enabled automaticaly when you log in. If you want to activate it, you have to use SQL command "SET ROLE ....".