In the event of an unsuccessful trade, you increase the amount of the following trade to cover the losses. As soon as a trade closes successfully, you go back to the original trade amount and start over.
For example, you open a trade for $1 and your prediction is incorrect. Next you open a trade for $2 and it fails again. Then you increase the trade amount to $5 and your prediction is successful. This way, you not only recover the losses on previous transactions but earn money. Usually, Martingale consists of 4 - 7 steps.