BEP-20 is covered in full in the entry BEP20 Token, which explains how the standard works, how tokens are created and the risks of using them. This page gives a short definition and explains the different spellings.
BEP-20 is the technical standard for fungible tokens on BNB Smart Chain, the smart contract network run by the BNB Chain ecosystem. It defines a common set of functions, such as checking a balance, transferring tokens and approving another address to spend them, so that wallets, exchanges and applications can handle any compliant token in the same way. The standard closely follows the ERC-20 standard used on Ethereum, which made it easy for developers to move projects across. Fees for BEP-20 transfers are paid in BNB.
The name is written in several ways, including BEP-20, BEP20 and "BEP-20 token"; all refer to the same standard. BEP stands for BNB Evolution Proposal. It should not be confused with the older BEP-2 standard, which belonged to the now retired Binance Chain, or with BEP-721, which covers non-fungible tokens on the same network.