Every N msecs, your audio interface wakes up the CPU to tell it to read and write X samples of audio data. The CPU (and the OS) in turn wakes up JACK to tell it do the same, along with any other processing that is part of that.
If it takes JACK N msecs to finish the task, the DSP load is 100%.
If it takes JACK 0.0000001 msecs to finish the task, the DSP load is very close to 0%.
If it takes JACK N/2 msecs to finish the task, the DSP load is 50%.
Values about 80% appear unstable/unusable on all 3 of Linux, OS X and Windows.