If you’re wondering what a “neutral density filter” is, it’s something that you put over your lens to block out a lot of the incoming light. Here’s a picture of the effect for the Wikipedia page on it.
So imagine that thing in the middle, but you can adjust how dark the picture is. At least that’s how I picture it; if you win be sure to let me know! I’ve taken some photos myself using an ND filter, as shown in this post. As illustrated there, one cool thing about this type of filter is the ability to open up your shutter for a longer period of time , without saturating the image. This lets water in motion look more like a sheet than individual drops.
It’s also probably good in an extremely sunny environment, like the beach, but some of this adjustment can also be done on a DSLR’s ISO setting. I don’t claim to be a good photographer though, so I’m sure there are other interesting uses.