A liquidity pool is the building block of a decentralized exchange. Liquidity providers (LPs) deposit pairs of tokens (say, ETH and USDC) into a smart contract. Traders swap one token for the other against the pool, paying a fee that is distributed pro-rata to LPs. The exchange rate is determined automatically by a formula (usually constant product, x*y=k) rather than by a traditional order book.
In a Uniswap V2-style constant-product pool, the product of the two token reserves is held constant: x * y = k. When a trader buys X with Y, the pool's X balance decreases and Y balance increases such that k stays the same. The exchange rate slides up the more X the trader buys, which is slippage. Newer designs (Uniswap V3 concentrated liquidity, Curve stableswap) use different formulas optimized for specific token pairs.
LP risks include impermanent loss (your share of the pool can be worth less than just holding the tokens, if prices diverge significantly) and smart contract risk.
Liquidity pools are how DeFi creates markets without an order book. They turned market-making from a professional activity into something anyone with two tokens can do. The LP model spread to lending, derivatives, and even prediction markets.
Pool flows correlate with on-chain activity tracked in the DeFi TVL premium endpoint.