It’s basically a smart contract that contains funds. In return for providing liquidity to the pool, LPs get a reward. That reward may come from fees generated by the underlying DeFi platform, or some other source.
Some liquidity pools pay their rewards in multiple tokens. Those reward tokens then may be deposited to other liquidity pools to earn rewards there, and so on. You can already see how incredibly complex strategies can emerge quite quickly. But the basic idea is that a liquidity provider deposits funds into a liquidity pool and earns rewards in return.