A ticket reopen happens when a customer replies to a closed ticket or manually changes the status to open through the customer portal.
A high number of reopens usually means that the agents aren’t making sure that the customer’s problem is fully solved before closing the ticket. But more often than not, customers reopen a ticket with a question regarding a problem unrelated to the original ticket because they want to deal with the same agent. So reopens, taken alone, are not a good way to judge the efficiency of the agents.