It's where you double your bet after every loss. So each time you make a bet it would put you 1 unit up overall when you win. For example, you bet $1 on red in roulette. If you lose, then you bet $2 next time. If you lose that you would bet $4. Let's say you win that time, then you've lost $3 and won $4 and are now up $1 overall. Obviously this system doesn't work because every single bet you make has a house edge so no matter how you bet you can't win.