What does an instrument's rollover value do?

The rollover is the maximum value that the instrument can display.

Max Instrument Reading (Rollover) 

A meter is used and there is maximum value that the meter can have. After the meter reaches that point it will then start over.  Example: 

Pump Run Hours: 

Max Reading: 999999 

Usage between work: 500

As readings are collected the difference between the values are calculated. In maintenance these values are then totaled together for total usage. The total usage is what determines when work will be triggered. In the above example, the instrument displays 6 digits. Once it reaches that max, it rolls over back to 0. With the rollover value entered, it knows to keep accumulating usage when it rolls over back to 0.