Cryptocurrencies are built on blockchain technology, which usually sets a maximum limit of tokens that can ever be created, or "minted". But not all of those tokens may currently be available for trading. The Circulating Supply is the number of tokens which have been minted, and are not locked or reserved in any form. They are in the hands of the general public, businesses, or are available on cryptocurrency exchanges.
Just as with supply and demand in other forms of financial markets, the circulating supply plays a large role in determining the value of a particular cryptocurrency. If the circulating supply is low and demand is high, this can increase the cost of each individual unit. On the other hand, a high circulating supply with low demand can decrease the price.
The actual circulating supply of a cryptocurrency can be hard to determine with absolute precision. Cryptocurrency creators or related foundations often hold a significant portion of the total supply which is not included in the circulating supply. But once it's sold or distributed in some other way, it is then added to the circulating supply. Loss, burning, or locking away of tokens can also reduce the amount of circulating supply.