Balance is the money a player can bet with at this moment: the funds shown in an online casino account, or the chips in front of them at a table. Every stake, win and loss changes it immediately.
It is often confused with the bankroll. The bankroll is the budget a player sets aside for gambling before they start; the balance is the live figure that moves up and down during play. A player might set a $500 bankroll, deposit $100 of it, and watch that balance double or fall to zero within one session.
On the operator side, the balance a player sees is often made up of separate wallets — real money, bonus funds and sometimes locked winnings — each with its own withdrawal rules. Showing these parts clearly, and updating them straight after every deposit or withdrawal, keeps players sure about what they can actually cash out.