In order to talk about a circular polarizer, you have to understand its purpose, what it works with.
Lets start with light. Light has many sources, could be the sun, a light bulb, or maybe even a reflection. Light travels out or emanates from its source. It radiates or travels out in a wave; it flows out randomly and in all directions. Light can be reflected, like off of a wall, glass, metal or even off of water. Imagine light traveling horizontally and vertically also in circular patterns clockwise and counterclockwise. By this I mean the light is almost on several different planes.
In photography, trying to capture a picture or image light is always present, in some form. You may or may not be happy with how the light is falling on your vantage point. It may be necessary to filter out the light to intensify work, it maybe necessary for clarity of an object. Your goal may be to enhance the color. It may be a goal to project more depth in an image. To do this, you would have to be able to control the pattern of light. Well thats exactly what a circular polarizer does. The word filter comes to mind. Allowing certain things in or keeping certain things out. In our case, it is light. With the circular polarizer, I am able to capture the light traveling in a certain pattern or it may even be thought of as having the abillity to block scattered light or atleast to be able to adjust it like a beam to make my enhancements. With the circular polarizer, I am in control of the tool in my arsenal to get the results that I need, feeling confident that I have achieved my goal in creating my perfect shot.