Service Level Agreement specifies the time limit within which a ticket has to be replied to and resolved. These limits are based upon the priority of the ticket. For example, your SLA rule can specify that urgent priority tickets have to be resolved within an hour. Low priority tickets, on the other hand, can take a couple of days.
SLAs set expectations for both agents and customers; it’s a basically a promise to the customer that you’ll solve his problem within a specified time frame. It’s a good practise to let the customer know when they can expect a resolution to their ticket as soon as they raise one.
If tickets aren’t responded to or resolved within the due by time, they become “overdue”. Which means that the promise made by the company to the customer when they raised a ticket is broken.
That’s why helpdesks let you send out escalation emails to supervisors/managers when an SLA is violated. This way, overdue tickets get immediate attention before the customer loses faith in the system.