An ERC-20 is a non-native cryptocurrency inside a programmable or smart contracts blockchain.
Because in smart contracts or programmable blockchains people can create any types of software programmes and dapps (decentralized applications), one type of software programme may be to create a new cryptocurrency, but that is not native to the network. These cryptocurrencies or tokens exist inside the blockchain as well, but they are not issued by the blockchain network. They are just created by developers to issue new cryptocurrencies for different purposes.
The term "ERC-20 token", which is used to create these non-native cryptocurrencies, stands for "Ethereum Request for Comment" or "ERC" and the number "20" is the number of that particular ERC. The ERC-20 definition of rules and ways to create non-native cryptocurrencies is now an industry standard.
Examples of these non-native cryptocurrencies or ERC-20 tokens may be Shiba Inu (SHIB), DAI, USDT, USDC, and HEX.