Means you're too late. Like if a guy likes you and then another one asks you out before him, you say to the first guy, "sorry, day late and a dollar short." So basically, you waited and screwed yourself over.
Guy: Hey, will you go out with me?
Girl: Sorry, day late and a dollarshort... Guy 2 just asked me out yesterday.
The crappy seats in the worst places you get when you are late some where. The ones in the first row in a movie theatre and in the back with the screaming babies at Church.