Answer
It's how long a stretch at a time that the compressor has been designed to supply air.
For example, a lot of hand tools require short bursts of air: impact wrenches, nailers, etc. Regular air compressors are fine for those tasks; the pump has time to refill the tank at intervals, and cool off in-between.
Other applications require a constant, steady source of air- like large sprayers and manufacturing tools. The pump has to be able to keep up the air supply without over-heating.
Sometimes these units are referred to as 'continuous duty', or '100% duty'.
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