What exactly does "noncritical disease" of "up to 20 percent" mean on heart cath report? why would this be low risk for heart attack/unstable angina?

Blood flow: For a stenosis to be significant, it has to affect blood flow. That does not occur until the stenosis reaches about 70%, at which point it is considered critical. Lesions around 50% are high risk due to potential risk for sudden occlusion. Lesions in the 20% range, when followed proved to be low risk. You should not have angna from this lesion, and it doesn't normally cause a heart attack.

Low % narrowing: At 50% diameter or 75% area narrowing there is a reduction f flow which is then a critical stenosis and may lead to ischemia of the end organ such as heart, brain, kidney etc.

Not hemodynamically : Significant, however, this does not mean no risk as 2/3 heart attacks are caused by blockages less than 50%. Talk to your physician about reducing your long term risk as you do have heart disease.