Polarising filters are probably the most important filters in photography. 

To understand when to use polarisers, it is important to understand reflectance.

Simply put, reflectance is defined as a "measure of the ability of a surface to reflect light or other electromagnetic radiation". 

While the polariser hasn't changed the amount of light leaving the surface of the an object, in this case bracken on the hillside, it has remained the same - but the polariser has prevented some of that light from reaching the lens and sensor.

In this case, the striking green landscape of Ireland's Wicklow Mountains, we can easily see the difference between the polarised and non-polarised image.