I tend to think of it in terms of EV. A marginal hand is a hand that has an expected value
of about 0. For instance, AJs UTG at a full table - I don't expect to show any huge profits with it in the long run, but I don't expect to be a significant loser either.
What's so interesting about marginal hands is that it's there that we can improve to find loads of money. Even a monkey can get it all-in with AA vs. KK, but as we get better we start to make better decisions in marginal spots. And that = $$.