The D'Alembert system is very simple. You increase your bet when you lose and decrease it when you win. The difference between D'Alembert and the Paroli system is that in D'Alembert you increase or decrease perpetually while in Paroli you always have the same base bet and only increase when you lose. D'Alembert is a twisted positive progression system that's absolutely awful for long term online blackjack games and was proven to be mathematically losing in games with negative expectation (read: all casino games).
You can play with it for fun but we definitely don't recommend trying it on your real money in a long term online blackjack game.