This is the most popular strategy not only in roulette but several other casino games too.
To play roulette using this strategy, find a table with small minimum bets and high maximum bets. Start small by wagering on red/black, even/odd, or on 1-18/19-36. In case the outcome is a win, wager the same amount, on the same bet as the initial. However, if the best is lost, double the stake, and bet on the same outcome as the previous bet.
There is another variant of Martingale called anti-Martingale, or reverse Martingale where players are supposed to increase the bet size if they win, and decrease the bet size when they lose.