In the radio programming world, AQH basically means "At any moment in time, this many people on average are listening to the radio station." The Nielsen Average Quarter Hour (AQH) metric was established decades ago, when people filled out diaries every 15 minutes to record their listening. It means the average number of people who listened for at least five minutes during a 15-minute time block.
In AQH terms, "100 QH" means that 100 people listened for at least 5 minutes during one 15-minute period. AQH simply averages multiple QH measurements.
Today we have much better technology for listener measurement than a diary filled out with a pen. The general concept is the same ("How many people are listening at any point in time") but the "quarter hour" aspect is no longer necessary. In Webcast Metrics, the Average Active Sessions (AAS) metric measures how many listeners a broadcaster has at any given second, and averages this for the selected time period. This measurement is more precise than AQH, but is otherwise comparable. Also, AAS does not require the listener to have listened for five minutes before they are measured.