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"Can you please tell me what does the three types of cold hardiness mean
-  low, medium and high in terms of actual temperatures the tree can
withstand.   Thanks"

It is hard to be that precise about cold hardiness in olives because so many variables come into play. The University of California cites temperatures below about 22º F as potentially lethal for young trees and branches on mature trees. The experience of some growers in Oregon, however, is that some varieties survive temperatures into the teens. There can also be a significant difference between the effect of cold temperatures depending on whether the onset is gradual or sudden because the trees can harden off if the temperature doesn’t fluctuate rapidly. Micro climates can also make a big difference; low spots where frost settles or areas with poor air circulation can have dramatically worse cold damage.

If you are in a location where cold temperatures might be a concern, avoid the notoriously tender varieties and don't plant in frost pockets. If you are really on the edge and there are no other olive trees in the area, it would be worth planting a few trees as an experiment. Leccino, Aglandau and Arbequina seem to be among the most cold hardy.