I look at them like this:
High Reach: 0-5% chance of getting in
Reach: 5-15%
Low Reach: 15-25%
High Match: 25-40%
Match: 40-60%
Low Match: 60-90%
Safety: >90%
Generally speaking: the lower the acceptance rate, and the higher the average GPA and test scores of admits, the harder it is to get into that school.