A CPL filter performs the same function as a polarizing filter when used on cameras. A CPL filter is a polarizing filter followed by a sheet of a material that has two indexes of refraction, one for each polarization direction. This is placed at 45 degrees to the polarizing filter and results in spinning the polarization of the light that passes through both filters.
There were some cameras whose auto focus sensors could be confused when attempting to focus on polarized light so they would not work with a simple polarizing filter. the CPL filter avoided this problem by spinning the polarization after polarizing the light. (I don't know why they bother with CPL on dashcams, they could just use polarizing filters. Dashcams are fixed focus cameras. I don't believe any dashcam would have trouble with polarized light.)
The purpose of the polarizing filter is to block light of a particular polarization. This is useful because light that glaces off water, glass or other shiny surface becomes somewhat polarized. By aligning the polarizing filter to block the polarized light reflecting off your windscreen, you will reduce the brightness of the reflection of your dashboard. It will also reduce the brightness of the sunlight reflected of other cars windscreens and give you a better chance of seeing what the other drivers are doing. The sky is also slightly polarized, and the polarizing filter will make the sky a darker blue. Tempered glass tends to rotate the polarization of light slightly, and can result in colored patterns in the sky that change with the orientation of the car when filming through side windows, or the rear window, but not the front windscreen. All windows in a car but the front windscreen are tempered glass. The front windscreen is not tempered, but is instead laminated glass. Tempered glass tends to shatter completely when broken, laminated glass cracks but does not collapse, so gives you a better chance of still being able to see where you are going when it gets broken.