Not entirely sure! It was a point of discussion on this community once. I personally think it’s one of those vague terms where you know it when you see it, and everyone has a vague idea of what you mean, but when you try to hammer down the specifics of what makes a cozy game and what is not a cozy game there’s a lot of disagreement.
I did find an article that tries to define what makes a cozy game cozy and think it makes a lot of good points.