What's a way to explain depression to people?

Simply: Use simple examples. "the blues, but much stronger" reference musical lyrics--"the blues" are called that for good reason. "it feels like grief--as though someone i love has died". And don't be afraid to reference poetry and literature. "o dark dark dark. They all go into the dark, the vacant interstellar spaces, the vacant into the vacant, the captains, merchants, eminent men of letters (t.S.Eliot.