The purpleair monitors measure microscopic particles in the air to help track harm from air pollution. Generally, the smaller particles penetrate deeper into the lungs and have more severe effects. The particles of greatest interest in the monitor report , PM2.5, are shown by clicking on a location dot (like this one).
The unit measures them frequently and sends a report about every 90 seconds. These are the readings shown when clicking on the real time tab. Then the short-term average is every ten minutes, other tabs show one hour and 24 hour averages. You can’t actually breathe these averages but they’re handy for watching trends.
There is a temperature and humidity sensor in the monitor, but a known error skews the readings. The “Current Weather” link may provide better information from weatherunderground.com.