The endowment effect is the valuing more of something just by virtue of already being in possession of it. So if i have a coffee cup and it's already mine, i might value it at five. If it's not yet mine, but it's the same coffee cup, i might only value it three. And that is problematic for a lot of classical economic theorizing because there shouldn't be any irrational utility gain from you simply being in possessionof it.