To order and pay for something before it's intended release date to guarantee a copy for you. Most stores will reserve a copy for that person upon its actual release, while others will only guarantee you to get one while it is in stock, but will not charge you since you've already paid. Online retailers will order at least the number pre ordered, and some will ship them to you in anticipation of delivery on it's release date.
The current world record for pre orders is Halo 2, sitting at around 20 million copies. This beat The Legend of Zelda: Ocarina of Time's record that was held since 1998.
John wanted a copy of Halo 2, and wanted one so bad that he wanted a copy guaranteed to him before it came out. He then went online, pre ordered a copy, and had one shipped to him when the game came out 4 months later.