The earths atmosphere has the effect of scattering sunlight, that is why our sky looks blue and not black like on the moon (since the moon has no atmosphere). During a very bright day, this scattered light will reduce the clearness of the sky and lower the overall contrast of a scene.
A side effect is that scattered light also becomes polarized. If you use a circular polariser filter on your lens, which can be adjusted to block out polarized light, you can darken the sky, make it clearer and increase the contrast in a scene.
No polariser filter - somewhat dull and hazy
With polarizer filter - bright and contrasty!