PancakeSwap is a decentralized exchange built on the BNB Chain (previously BSC and Binance Chain) instead of Ethereum. It enables its users to swap BEP-20 standard tokens easily.
BNB - (Binance Chain) a blockchain platform developed by the popular centralized cryptocurrency exchange to support Binance DEX, a decentralized exchange that allows users to directly trade with each other.
BEP-20 is a token standard on BNB Smart Chain that extends ERC-20, the most common Ethereum token standard.
(Correct me if im wrong, because im a newbie too)