What's a Primary Guardian? What's a Secondary Guardian?

Only one Adult account gives Gold Membership to a Kid account: the Primary Guardian. Often this is the parent, but sometimes it's a teacher, coach, or school.
 

However, more than one adult can access a student's account (to review their games, or monitor their activity on the site). Any other adult account with access to a kid account is a Secondary Guardian. They still have full access to the kid account, they just didn't purchase the Gold Membership.