In short
MTBF is the average operating time between two failures and MTTR is the average time spent repairing a failure; both are key maintenance metrics.
As MTBF (Mean Time Between Failures) rises, equipment becomes more reliable; as MTTR (Mean Time To Repair) falls, failures are handled faster.
For these metrics to be meaningful, failure start and end times, failure causes and interventions must be recorded consistently. That is a core function of a CMMS or other maintenance management software.