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.

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