Card counting is keeping a rough tally of high cards versus low cards as they leave the shoe. When the remaining deck is rich in tens and aces, the player’s edge ticks positive — more blackjacks, more dealer busts, more successful doubles. You raise the bet. When the shoe is junk, you bet small or leave.
Hi-Lo is the classroom count: plus one for 2–6, minus one for tens and aces, zero for 7–8–9. Convert to a true count by dividing by remaining decks. The rest is discipline and a face that does not change when the count does.
It is legal in the United States. Casinos are also legal when they ask a counter to take a walk. Online, continuous shuffle machines and six-to-eight-deck shoes make counting a study project, not a living. Learn it because it teaches you how the game is built. Do not learn it because a banner promised you a yacht.
Start with basic strategy — counting a mistake-filled chart is lighting money on fire with extra steps.
General information, not advice. Gambling involves risk. See the disclaimer.