What does a high efficiency gap really say about gerrymandering?

[Kenneth70]

I was reading an article about my state's new congressional district map, and it kept mentioning the efficiency gap as a measure of partisan fairness. I think I understand the basic math, but I'm confused about what a "high" or "low" score actually means in practical terms. Does a high gap always indicate intentional gerrymandering, or can it sometimes just be a result of where people naturally live?