The ERC20 token standard contains the requirements that govern how new cryptographic tokens can be launched on top of the Ethereum blockchain.
Similar to how the HTTP protocol defined the internet, ERC20 is a protocol that defines a set of commands that a token should implement. ERC20 is not a technology, software or piece of code. It is a technical specification. If a token meets the specs, it is an ERC20 token. To know more in detail, we recommend that you read here.