It will keep ticking as long as the key is turned on, but you can reset it to zero just like with the odometer.
It's just a timer to keep track of whatever you want.

 

When I used to work in the Oilfield years ago.....we used hour meters on all the Semi's and regular P/U's, as both were running nonstop, specially in the winter sitting on a lease...I believe Ford Superdutys have this option as part of the ODO.....even though the Pickups were turned over every 200,000kms, the hrs on these trucks were ridiculous.....always well maintained though....