The ERC-20 introduces a standard for Fungible Tokens, which means that each Token has a trait that makes it identical to another Token (in terms of type and value). An ERC-20 Token, for example, functions similarly to ETH, meaning that one Token is and will always be equal to all other Tokens. It includes features such as the ability to transfer tokens from one account to another, as well as the ability to view an account's current token balance and the total supply of the token available on the network.
It also has various other features, such as approving the use of a certain quantity of token from one account by a third-party account.