A puddle is a small pool, such as one that forms on a sidewalk after a rain shower. A little boy with new rain boots might decide to jump in every puddle he sees on his way to school.
It's charming to see a puddle on a brick walkway or to watch a duck wash itself in a puddle. It's less charming to find a puddle in your basement after three days of rain. You can use puddle as a verb, too, to say, "I'm watching your cereal milk puddle in the middle of the table and wondering when you're planning to clean it up." Puddle comes from the Old English pudd, "ditch or furrow."