A liquidity pool is a crowdsourced pool of cryptocurrencies or tokens locked in a smart contract that is used to
facilitate trades between the assets on a decentralized exchange (DEX).
Liquidity pools aim to solve the problem of illiquid markets by incentivizing users themselves to provide crypto liquidity for a share of trading fees. This means users can simply exchange their tokens and assets using liquidity that is provided by users and transacted through smart contracts.
Source: Gemini.com