In cryptocurrency, max supply refers to the maximum amount of tokens or coins that will ever be in existence. We can take the example of Bitcoin, the maximum amount of Bitcoin that can ever exist is 21 million. While the minority is already mined the rest are still in circulation. Once Bitcoins are exhausted, they can’t be produced and mined.
The reason Bitcoin is expensive is related to its limited supply and the large demand. The case is different with Ethereum. Ethers (tokens of the Ethereum network) have an unlimited supply since their creation is limited to 18 million per year.