The term used by guys to address a girl who's easy to get; a girl who is not that hot but who guys hook up with anyway because they don't have to work for her.
Think of your least flavor of ice cream. If someone offered it to you for free, would you take it? Probably. If you had to pay for it, would you still buy it? Probably not. This is how guys think of girls. If there's a flavor of ice cream they don't have to work for, they'll take it. If it's not worth working for, then they wont.
Guy: "So you going to get with sally tonight? She's not that hot."
Other guy: "Yea I know but that's some free ice cream right there, she's dying for my dick."