A pool is a small body of water, like the tide pools that form on rocky beaches at low tide, or the town pool where you take swimming lessons.
Pool is a common word with many different meanings. Pools are natural and human-made collections of water, from deep plunge pools at the base of a waterfall to rectangular reflecting pools in parks. A pool is also a supply of something, like workers or money. As a verb, pool means "to combine": "Let's pool our resources and go to Disneyland!" Then there's the game called pool, whose name comes from players vying for a pool of cash.