MTBF measures the time between failures for devices that need to be repaired, MTTR is simply the time that it takes to repair those failed devices. 

In other words, MTBF measures the reliability of a device, whereas MTTR measures the efficiency of it's repairs. 

https://www.logicmonitor.com/blog/whats-the-difference-between-mttr-mttd-mttf-and-mtbf#:~:text=MTBF%20measures%20the%20time%20between,the%20efficiency%20of%20it's%20repairs.