A CPL filter is a circular polarising filter.
Polarising is used to filter out the stray light that gets bounced of reflecting surfaces. Two uses where the results are obvious would be a sky with lumpy clouds or a fish pond. The clouds would look more vibrant and, lined up correctly, the fish would be visible beneath the water instead of the reflection of the sky .
'Circular' means it's possible to twist the filter independant to the lens so you can adjust the effect without twisting the lens and body to suit.
They're great filters for outdoors. Polarising sunglasses do the same job for your eyes so you can test the effect before you spend money .