Free Lunch game mechanic is a mechanic where you receive something for free. In World of Warcraft there was an event with the new released Warcraft film. During the release of the film as soon as you log-in you would obtain these 2 items for free. You’ll get a notice and an achievement and these items would be sent to your in game mail.
Although these things are also commonly known as scam messages as well, where you receive an e-mail asking for your details because you have been qualified to test out new content in game which on most occasions can result in your account being stolen.