Circulating supply is the approximation of the number of coins that are circulating in the market. If a cryptocurrency is mineable (like bitcoin), new coins can be created gradually via mining.
Even though the circulating supply of Bitcoin (BTC) should be over 18 million coins, it is estimated that around 4 million BTC have been permanently lost, placing the true count closer to 14 million. These BTC have been lost mostly via accidents, like sending coins to an irrecoverable address or losing access to a wallet where funds are stored.
Besides that, some cryptocurrencies have already minted all the coins from the start. These are usually crypto made with a proof-of-stake (PoS) and don’t require mining.