Max supply is the hard cap on the total number of tokens that can ever be created for a cryptocurrency. Once max supply is reached, no new tokens can be minted. Bitcoin's max supply of 21 million is the most well-known example.
Not all cryptocurrencies have a max supply. Some tokens have inflationary models where new tokens are continuously minted, often to fund staking rewards or ecosystem growth. In these cases, the max supply may be listed as unlimited or undefined.
A fixed max supply creates scarcity, which is often viewed positively by investors. Combined with token burn mechanisms, it can even lead to a deflationary supply model where the total number of tokens decreases over time.