A liquidity pool is a smart contract where a pair of tokens is locked and made available for swaps. Any pair of tokens can have liquidity pools created for them (mSOL/SOL, SOL/USDC, MNDE/SOL, etc.). This smart contract allows anyone to exchange one of the token of the pair for the other, in exchange for a small fee distributed to the liquidity providers, people who made their liquidity available in this smart contract.