Light reflecting off non-metallic surfaces becomes polarized — its waves align to one orientation. A circular polarizer rotates to block that orientation selectively, which removes the reflection while leaving everything else. It is one of very few effects that genuinely cannot be replicated in software, because the information was never captured.
The results are dramatic. Glare vanishes from water so you can see the riverbed; reflections lift off shop windows and car paint; wet foliage stops looking blown out and reveals saturated green. Blue skies deepen because atmospheric scatter is itself polarized, and the effect is strongest at ninety degrees to the sun.
There are costs. A polarizer eats one to two stops of light, and the sky effect is uneven across a wide-angle frame — pointing a 16mm lens upward can produce a visibly darker band. It also needs rotating for each new composition, which is why many shooters keep it for specific problems rather than leaving it mounted.