The Canon 95mm Protect Filter offers protection to the front element of the lens from dust, scratches, moisture, and other environmental elements. This filter doesn't affect the original coloration of the image. The protector filter also ensures added safety against accidental drops. You can fit other 95mm ND or polarizer filters to the accessory thread.
When placed in the optical path, many filters block a certain amount of light from reaching the lens. Filter factor relates to how much exposure compensation is required in order to adjust for this. Most cameras have TTL metering and will be able to do this automatically.