Total number of coins available for use in the market, for the public, and trading.
The circulating supply of a cryptocurrency is almost always lesser, by some distance than its maximum supply. The remaining supply is slowly released over time through mining, liquidity rewards, or some other mechanism embedded in the token design.
Circulating supply indicates the quantity of the tokens available relative to the quantity already created (total supply) and the quantity that will ever be created (max supply). Circulating supply can be a good indicator of a token's scarcity but doesn't account for lost or stolen tokens.