All light waves have an orientation they oscillate in, but most light sources emit light in all orientations, such as the sun. We call this unpolarised light since we get a saturation of waves in all angles.
But when light reflects off surfaces, depending on the material, it has a chance of being reflected in only one orientation. The more reflective a surface, the more uniform the orientation in which the light will be reflected.
For example, if unpolarised light from the sun reflects off a horizontal pane of glass, almost all of the reflected light will now oscillate in the horizontal direction. This is where polarising filters come in.
While there's a two different types of polarisation and polarising filters, they are relatively similar in their effect. Essentially what a polarising filter does is completely block on specific orientation of light. This has the added benefit of blocking other light waves proportionally to their angle against the filter.
When we orient the filter horizontally, that light will be let through but light bouncing off a vertical surface like a wall will not be allowed through. Your sunglasses are polarised in the vertical orientation so as to block the light reflected off horizontal surfaces like car windows and the road.
You can see very noticeably below the effect that changing the angle of your polarising filter has on the scene. In the tree, the horizontal leaves will become a lush deep green due the filter cutting out the bright reflected light, leaving only the green emitted light from the leaves.
THIS is why using a polarising filter for landscapes is so important to bringing out the rich, deep colours of the scenery and is one of the main tools at your disposal when making landscape photographs.