ERC-20 is covered in full in the entry Ethereum Request for Comment 20 (ERC20) Token, which explains the standard's functions, its history and its role in token sales and decentralized finance. This page gives a short definition and explains the different ways the name is written.
ERC-20 is a token standard for fungible tokens on the Ethereum blockchain, meaning tokens in which every unit is identical and interchangeable, like coins of the same currency. Proposed by Fabian Vogelsteller and Vitalik Buterin in 2015, it defines a small set of functions, such as checking a balance, transferring tokens and approving another address to spend tokens on the holder's behalf, that every compliant token contract must offer. Because wallets, exchanges and applications can rely on this common interface, any ERC-20 token works with them without custom integration. Most stablecoins on Ethereum, such as USDC and Tether, and most governance and utility tokens follow it. Unique items use other standards, such as ERC-721 for NFTs.
"ERC-20", "ERC20" and "Ethereum Request for Comment 20" all name the same standard; ERC stands for Ethereum Request for Comment, the category of Ethereum Improvement Proposals that defines application-level conventions.