Please explain an idiom for me !

Dear teachers,
Would you mind explaining the idiom :"Cry me a river" for me? I have heard that idiom so many time such as on the TV, the Internet and so on, but till now, I don't really know what it means.
Your faithfully,
A student comes from Vietnam. EMAIL REMOVED - Send PM to This User Instead

[Ouisch]

The original phrase came from a song title, "Cry Me a River," and referred to someone crying so much they could fill a river with their tears. When used as an idiom, it is a sarcastic way of telling someone "you can moan and complain all day long, but you won't get any sympathy from me."

Ex:
Bob: "My flight was terrible. There was an error in my reservation and I was unable to get a seat in First Class."
Fred: "Oh, cry me a river. My original flight was cancelled and I had to spend the night sitting in a chair at the airport so that I could catch the first plane heading out this morning."