he answered your question in regards to an air compressor. Since I am an HVAC guy, I will answer it in regards to that, since you were not clear on the application.
In regards to the compressors on Chillers, unloading refers to the reduction of the cooling capacity of the chillers due to the cooling load in a space being satisfied without having to run the chillers at maximum capacity. Chillers are sized for worst case based on ASHRAE design conditions. Since this case doesn't happen but a few times a year, the rest of the time, you only have to run your chiller at part load. To accomplish this, with a reciprocating compressor on a chiller, well, you don't. What you do is put multiple compressors that add up to the total load of the chiller. An example would be, if you had a 100 ton chiller, you put 4 25 ton compressors. What that allows you to do is completely turn off one compressor and runt at 75 tons, or 75% capacity. Reciprocating chillers have a limitation as to how much the can unload, and that is the capacity of the smallest compressor. That is why you try and get as many compressors as possible, so you can unload to a smaller capacity, especially in areas where the building cooling load varies greatly from minimum to maximum. There are limitations for this due to size and cost. To achieve better unloading capabilities, you would want to use a rotary type compressor. Screw type compressors can unload down to 10% by controlling the speed at which the compressor rotates.
Hope this helps if you were looking for this info.