Maximum supply is the maximum amount of coins that will ever exist in a certain cryptocurrency. Once a cryptocurrency reached this maximum supply, no new coins will be produced or mined. Most cryptocurrencies will outline the production of their coins whether in a general or detailed timeline. These outlines usually exist in the coins white paper or website.
In Bitcoin, the maximum supply is fixed at 21 million coins. Other cryptocurrencies, such as the ether of Ethereum, does not have a predetermined fixed supply as it is constantly produced.