419 Scams are advance fee frauds, a form of confidence trick. The number, "419", refers to the article of the Nigerian Criminal Code (part of Chapter 38: "Obtaining property by false pretences; Cheating") dealing with fraud.
The victim receives an email making an offer that would allegedly result in a large payoff for the victim. Eventually the scammer then introduces a delay or monetary hurdle that prevents the deal from occurring as planned, such as "To transmit the money, we need to bribe a bank official. Could you help us with a loan?"