It's a betting system where you essentially double your bet every time you lose in a game where you have as close to a 50% chance of winning as possible.  However, essentially all this does is provide the gambler an opportunity to make small gains while opening themselves up to the risk of a catastrophic loss in the event that the highly improbable yet inevitable event occurs of them losing a certain amount of times in a row.  It is not a 100% sound method by any means.