There are a few ways that a ticket can be closed. The most common is that you've fixed the vulnerability, and a new scan has verified the fix. In this case, the ticket will be automatically closed for you.
You can also close the ticket manually. The ticket will then have status "Closed/Resolved (Unverified)" and will have to wait for the service to verify it on the next scan. When service verifies that the vulnerability has been fixed the ticket will be closed with status "Closed/Fixed".