To put it simply: Hill height on the map means EXACTLY as many meters, as you like
When building my maps, I usually tend to maintain "Each height level = 20 meters". Works pretty well in creating realistic lines of sight.
I use often the fact that height "+4" or "-4" creates cover (atleast on level ground) to make sunken/raised roads etc.
Other than that, as I said, one hill level (ie height 10) is anything you like it to be.