Examples of such devices range from self-resetting fuses (where the MTTR would be very short, probably seconds), to whole systems which have to be repaired or replaced.
This does not mean the supplier is guaranteeing to have the system up and running again within 24 hours (or 7 days) of being notified of the failure.
It does mean the average repair time will tend towards 24 hours (or 7 days).
The former indicates that the supplier will acknowledge a problem and initiate mitigation within a certain timeframe.
Some systems may have an MTTR of zero, which means that they have redundant components which can take over the instant the primary one fails, see RAID for example.