I'd assume you mean something along the lines of "Kann man essen". That would be shortened version of "Das kann man essen", "Die Suppe kann man essen" or something similar.
For describing food, it's an average judgment. You can eat the food, it's not unedibely bad, but it's not fantastic either. As you said, "you can eat it", but not too much more.
It's also used in a tounge-in-cheeck way to describe food that actually is really good or that the cook is very proud of. In this case, you're basically pulling the person's leg by describing the food as average.
An even stronger phrase would be "Der Hunger treibt's rein", which is a shortened form of "Der Hunger treibt es hinein" (roughly "The hunger gets it into you" or "If you're hungry enough, you'd eat even that"). But I'd use that one in jest only with people you know very well.