So ND of course refers to neutral density. A neutral density filter reduces the amount of light coming into the camera without changing the quality of the light. Filters like most terminology with a camera are divided into stops. Think of stops as sections in a pie. You start out with a whole pie, put an ND 2 over your lenses and you cut the pie or the amount of light coming into the camera by half. Put an ND 4 in front of your lenses and you now have 25% of the pie/light.
So as you can see everything is either haved, or doubled when you refer to stops. Well not everything, as cameras and filters can do fractions of a stop like 1/3. But generally speaking , stops refer to cutting the light in half or doubling the amount of light So in the end E8 would be 3 stops of light removed. The reason you bother with ND filters is because by reducing the amount of light coming into the lens you don't have to manipulate things like aperture, or shutter speed, or iso to get an acceptable picture. On some very sunny days, it would not matter how you manipulate the camera as it would still be too much light. So the ND filter allows you the flexibility to get an acceptable picture by cutting back on the light.
CP is a circular polarizer. Think of your sunglasses. Without your sunglasses looking out on a sunny day light is bouncing off everything in a very scattered, haphazard fashion. The sunglasses allow the light to become more uniform so it adds greater clarity. That's what a good pair of sunglasses do and that's what the CP would do. I have often used a polarized filter when I'm shooting at the beach. What's so much light reflecting from the sand and the water, polarizing filter helps to bring out the colors and Clarity so that everything is not washed out.
You can look up stuff online or read books but the best way to learn is just take your filters out and play with them. Take a pen and paper and markdown all the parameters before you fly. Pick a sunny day, and write down the shutter speed, the ISO, the aperture if your particular drone allows you to adjust it. Take a flight, come back repeat the same flight with the various filters. Play back the video and pics and see what you like and what you don't like. If you do this Hands-On approach you'll learn very quickly how to improve the quality of your media.
Sorry, Jefti, even though your explanation of what ND's are and how light intensity is measured is fairly accurate, your suggestions for how to use them on your drone are not correct. A drone, or any other camera, does not need a filter to diminish the light level to make a good picture. In fact, it is usually the opposite with drones. You want shorter shutter speeds and be able to control a smaller aperture for greater depth of field. A lot of light provides you those opportunities. ND's have two basic purposes on a drone. By diminishing the light, you can achieve a shutter speed of 1/60, which is the generally recommended SS for capturing 4K video. Mid day light will often not allow you to get that setting even if you stop down the aperture to it's smallest size, which is not good for image quality. For still photography, you may want to capture a waterfall or river, and get that silky white effect that is done by using a longer shutter speeds such as 1/4 or 1/2 second, or longer. Again, you can not get that setting in regular daylight conditions. Using an ND will let you do that.
Using a CP on a drone is tricky and not always successful. CP filters work best when the direction you are facing the camera is 90 degrees to the light source. Keeping your axis near parallel to the light source will negate the effect of a CP, and you will lose around 1-1/3 stops of light for no gain (which is how much light a CP robs you of). And, those filters rotate one element of it's glass to adjust the effect. You can not do that while flying. And further, capturing wide angle images that include blue sky while using a CP will always give weird effects, as the amount of polarization will change from left to right as the angle of the light changes within the image. So your sky will be much darker on one end and gradually get lighter towards the other end of the sky. This always looks bad. Test this by looking thru a CP on a regular dslr camera and turn right and left. You will see this effect clearly.