Mean time to resolve (MTTR) is a support metric: the average time between a problem being reported and it being fully resolved. It is calculated by adding up the resolution times of all tickets in a period and dividing by the number of tickets. MTTR is sometimes used to mean "mean time to repair" or "recover", so check how a provider defines it.
It is usually measured from a helpdesk or ticketing system. Providers often track it alongside response time (how fast someone first replies), first-contact resolution (the share fixed on the first call) and SLA compliance. Averages can be skewed by a few very long tickets, so a median or a breakdown by priority is more useful.
Retailers and hospitality businesses use it to judge whether support is meeting the business need. A till fault at the start of service matters far more than a request for a new email address, so resolution targets should differ by priority level and be written into the service level agreement.
Pitfalls include a provider closing a ticket before the problem is actually fixed, ignoring tickets that need parts or site visits, and comparing numbers across providers who define them differently. Ask what is included and look at actual priority-one cases.
Tracking it shows whether support is improving and helps decide where to invest in more reliable equipment or spares.