AHI meaning? Averages with used machine?

What does AHI mean? I know what it stands for, but what is it measuring?
And how are you supposed to know what your averages mean if you have a used machine and the guy before you has their 26.5 hours averaging in with your measly 12? Is there a way to set the total hours on the machine, so that only mine info is being considered?
I'd like to know how effective my treatment is based on only my info.

AHI (Apnea Hypopnea Index) is the average number of apneas (complete cessation of airflow) and hypopneas (decrease in airflow) per hour. An AHI under 5.0 is considered "normal".
Perhaps if you share your machine make/model, someone can tell you how to reset the running totals.