If you do something passably, you do it fairly well — not badly, but not really well either. It sits in the middle: a passably good meal, a passably funny film. The feeling is mild, even a little cool. You are not excited; you are saying it will do. Though actually, it can also sound faintly kind, as if you do not want to be harsh: “She sings passably” is softer than “She sings badly,” but it is still not praise. It is more at home in writing and careful speech than in very casual talk, where people might just say “okay” or “all right.”