This tab is used to define which roles the membership should contain. When the membership is assigned to a user, it will grant all permissions allowed by the specified roles until it expires for the given user.
To add roles, click the Add roles button and check the boxes next to the appropriate roles in the displayed selection dialog. Only roles that belong under the same site as the membership can be chosen (global memberships may only contain global roles). Roles can be removed from the membership at any time using the checkboxes in the list together with the Remove selected button.
Additional information can be found in the Developer's Guide -> Development -> Membership, permissions and security -> Memberships chapter.