In addition to the excellent answer from Sara:
UMass coherence measure how often were the two words (Wi, Wj) were seen together in the corpus. It is defined as:
D(Wi, Wj) = log [ (D(Wi, Wj) + EPSILON) / D(Wi) ]
Where:
D(Wi, Wj) is how many times word Wi and word Wj appeared together
D(Wi) is how many times word Wi appeared alone in the corpus
EPSILON is a small value (like 1e-12) added to the numerator to avoid 0 values
If Wi and Wj never appear together, then this results in log(0) which will break the universe. EPSILON value is kind-of a hack to fix this.
In conclusion, you can get a value from very big negative number all the way till approx 0. Interpretation is the same as Sara wrote, the greater the number the better, where 0 would be obviously wrong.