House edge. Even with perfect basic strategy, the house wins in the long run. Under common 6-deck rules, its edge is roughly 0.43-0.5% of total money wagered.
This site is a blackjack basic-strategy trainer for general education and practice. It is not a guarantee of the correct decision in every real casino situation.
Casino rules, promotions, deck penetration, table restrictions, composition-dependent play, and local variations can all change the exact best move or the expected return.
Blackjack involves risk, variance, and the possibility of losing money even when every decision is theoretically correct. Do not treat this tool as gambling, financial, legal, or professional advice. Only gamble where it is legal, if you are of legal age, and never with money you cannot afford to lose.