What does ACH mean?

An air change per hour translates to how many times an air purifier can clean a room in one hour.
For example, 2 air changes per hour shows that an air purifier can clean a room once every 30 minutes.
An allergy and asthma sufferer needs at least 4 air changes per hour,

which means an air purifier can clean a room at least once every 15 minutes.