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Nice you ask... a quick answer is like an automated market where people put in "liquidity" and then everyone can swap back and forward without the need of a trade (match)... because the pool will dictate the price via the amount of tokens swapped in each direction.

You can read a couple triplet posts I have done... if you want more detailed information. They are not too long.

https://peakd.com/hiveengine/@forykw/understanding-liquidity-pools-on-hive-engine-into-basics

Let me know any feedback if any.