In Poker, a backdoor is a situation where the Player is missing two additional cards to achieve the desired combination. Accordingly, it is a draw, i.e. a poker hand that is not yet finished.
In principle, a backdoor can be differentiated more precisely between backdoor flush draws and backdoor straight draws. This only refers to the "desired hand" to which the player is still missing two cards. A backdoor flush draw is when a poker player has two cards of clubs in his hand, for example, and another card of clubs is revealed on the flop. In this case, the player is only two cards short of a flush. This is a backdoor flush draw.
In general, it can be said that the strength of a backdoor draw is relatively low. However, with a bit of Luck, there is the possibility of completing one's combination in the further course of the game (e.g. with the turn and river card), so that in this case one can score with a strong hand such as a flush or a straight. If you already have three cards of the same suit, i.e. a backdoor flush draw, the probability of actually achieving a flush with the next two cards is 4.2%.