The amount of coins (tokens) that are publicly available and circulating in the market. The circulating supply of a cryptocurrency can increase or decrease over time. This number can change over time, as new coins are created or old ones burned.
Circulating supply is important because it reflects the total number of coins available to the public. Moreover, the circulating supply of a cryptocurrency is often used for calculating market capitalization of the coin.
Circulating supply is the supply in the law of supply and demand. If it is high and demand is low, prices of respective coins will depreciate. If supply is low and demand is high, then the coin prices will appreciate, raising the value of the coins.