When a user joins an association, their newly created membership is in an "unchecked" state. This lets an administrator check only the memberships that are appropriate, differentiating them from fraudulent or unwanted users.
If the organisation has "membership pre-approval" enabled, unchecked members can't make payments for their selected membership items or addons until an administrator checks them.
To check a membership, navigate to Members → Unchecked, select the members, and choose "check members" from the batch process options. Alternatively, select a member, click the View tab, select the checkbox marked Checked, and click save.
Once marked as checked, a membership cannot be set back to unchecked.