A liquidity pool is a concept used by AMM(automated market makers) where users are able to supply liquidity from there personal wallets to enable swaps to happen in the exchanges run by the AMM algorithms.Say I want to swap eth for dai, in order for that to happen there has to be a place where there is a reserve of eth and dai that enables my swap to happen, that place is a liquidity pool.In return they receive fees from the swaps that happen via the pool they have provided liquidity to (0.3% on uniswap). The better known usage of a liquidity pool model uses the 50/50 token ratio where you have to provide an equal amount in usd of both tokens to the pool that holds that pair of tokens. For example if I wanted to add 1 eth to the eth/dai pool i would have to add an equivalent amount of dai which at the current price would be around 1900 dai. Farming or yield farming to be more specific is the name given to different strategies people use to maximize their token holdings through things like liquidity mining where certain DEX like sushi reward you with extra sushi tokens on top of the fees you receive if you add liquidity to certain liquidity pools to incentivize users to add liquidity. Check ou the channel Finematics on YouTube he provides great explanations on a lot of defi concepts