As an undergrad, I struggled to understand what this phrase meant. In the context of a scientific experiment, it refers to a set of criteria of things that are required, but not enough to fulfill the desired outcome. This needs an appropriate analogy. And I've thought of the best one.
"Nice." As a characteristic, it is a great thing to say about someone. It's much better than "inconsiderate," "cocky" or "self-assured" (minus the self...ured) However, if this is the only thing that comes to mind... you don't think very highly of the person at all. They are lacking of substance!
I was thinking about relationships, residency and beyond and I found this thought most appropriate to both. I haven't received any evals that only say "nice," and I hope I never will. I do hope however, that I can clarify the phrase "necessary but not sufficient" to someone with the word "nice."