It's a variable ND polarizing filter.
The ND filter has 5 stops; there are also double-stop filters, 10 stops (ND1000, 1024 to be exact).
The polarizing filter eliminates light reflections on surfaces, cars, water, and glass, and also enhances the colors of the sky, leaves, asphalt, etc. (for more information, search the internet).

Personally, I use it for video, so as not to lose depth of field by closing down the aperture (f9 and up; now I can go to f2.3 without burning frames due to excess light). I haven't removed it from my 35mm lens in a while; I'm delighted.

The filter is made of metal and glass, presented in a hard plastic case for storage. You can see the protective and anti-reflective coatings in a greenish color, and you can put your finger on it, and it holds up quite well without smudging.
The lever adjusts the filter density, and the other ring moves the polarizing filter. You have to pay attention to this because you might accidentally move both.

In the images, you can see the filter attached to an adapter (49mm to 67mm step-up). This is because I have 67mm and 49mm filters. My advice is to buy larger filters and then step-up adapters.

As for image quality, it's excellent: there are no casts, no chromatic aberration, no noticeable loss of definition, and no appearance of the terrible X that Gradual ND filters show.

I have a very expensive polarizing filter, and I can barely notice the difference with this one.

On the downside, although I already mentioned it, be careful when handling one of the rings so as not to move the other.

**If this has been helpful to you, remember to give it a thumbs-up vote; this way, we'll all be helping each other.** :)

Bought this for my 35mm f2 which I am using on a Canon RP. I wanted it for slowing down the shutter speed for waterfalls I'll visit this summer, as well as deepening the blue in skies. It works as advertised. Nice metal build quality that feels well made. I like the stiffness on the variable ND part. It stays where I set it, and so far it's working great at reducing the light coming into the camera, allowing me to use longer shutter speeds to get the flowing water effect I was after. I usually use the polarizing part of the filter on the lowest ND setting. I'm adding two photos that I think show the polarizing effect pretty clearly. There is some colour difference between using the filter vs not as well as at different filter settings (rotations). I think this is pretty standard, at least I've observed it in the polarizing filters I've used in the past. I've been satisfied dealing with colour shifting using WB and calibration adjustments on raw files in Lightroom. I don't see any degradation in sharpness. I'm pretty picky and do view files at 100%. Very glad I purchased this filter combo. Great value for the money! I'll also add that I accidentally used it once over a thin UV filter and was glad to not see any vignetting.

Great quality of coating. Doesn’t ruin pictures neither on min nor max position.

Has two cons:
No cap can be attached.
ND ring easy shifts CPL, gotta hold CPL when adjusting ND.

In general - great filter.

Picture of the solar eclipse taken with this exact filter.

This polarizing and neutral density filter pleasantly surprised me. It's not overpriced, and the photo quality is pretty good. I've taken several photos with the different levels of light reduction, and the results are good. If you like silk effects in water, this is a must-have filter.
It also has a polarizer that eliminates reflections on glass surfaces.

Great kit for long exposure. If your shooting in bright sunlight it's a great help.

Very easy to use and produces great quality images. Being able to adjust the exposure without removing and adding filters makes shooting so much quicker. Image quality is great.

Another well designed and implemented filter. Perfectly fit the bill and does what is advertised.

Box for carrying the filter is nicely crafted as well.

And, microfiber cloths are well appreciated.

This filter is excellent if you want the convenience of a dual filter (CPL and ND) and a continuously variable ND filter. It functioned flawlessly. Be aware though that it's a little more difficult to adjust because of the physical structure of the lens but that's not an issue with the design or this company. It's just the nature of combination filters like this. Also be aware that this filter is of course, twice as heavy as a separate filters used individually. This means it can be a bit more difficult to balance on a lens on a smartphone if you use a gimbal. I still was able to though using offset weights on my gimbal.

If you are looking to get into car photography or are already in it, this is a must! The build quality is 10/10 and the image quality stays the same. Yes their might be a tiny change but unless you are a pixel peeper you won't notice the difference. It's nice to change, and eliminate the glare of the sun on cars that are on the body but also the windshield. The only thing that is slightly annoying is when I move the ND it sometimes moves the cpl slider and can change and image so be aware. But over all if taking pictures of car I set the ND first, then rotate the cpl filter. I usually leave this filter on all the time no matter what I shoot since it really does not affect the image quality, and have the ND set to Min. I highly recommend!

Works great, has smooth action and kills two birds with one stone being able to spin and kill reflections. Only complaint is incompatibility with many lens hoods but oh well, better to reduce diffraction and hit your target than to eliminate diffraction and have to jump through hoops to get a decent shot.