There are a ton of lens filters out there that can do some pretty cool stuff to your images, most of which you'll probably never need. However, there are a few of them that are almost a required purchase, because not having them on you could mean disaster if you find yourself in unfavorable shooting conditions. And one of those all-important filters is the ND, or "neutral density" filter. These things, which are often called "sunglasses" for your lens, are used to reduce the intensity of light coming through the lens without affecting the quality of the color.
If you're a beginner and literally have no idea what the hell to do with an ND filter, then you should definitely take a look at this video from Peter McKinnon. In it, he gives you a two-minute crash course on what ND filters are, what you should use them for, as well as what you should use them on.
Hopefully now you know a little bit more about why you might want to consider getting your hands on an ND filter, but before you do, make sure that the camera you're using doesn't already have them built-in. The camera I primarily use for projects has built-in ND filters, so the only ones I ever had to buy were for my old 35mm film camera that I used to use to take stills. However, McKinnon makes a good point—make sure to get ND filters for all of your cameras. Do you have a drone? A GoPro? What about your smartphone? There are ND filters custom built for all of them.
If you want more advanced advice on how to utilize an ND filter, check out this three-part video series by B&H. It covers everything from different types of ND filters to judging the quality of a filter before you buy. It even talks about something that most new filmmakers encounter at some point, which is having to capture a shot that moves from dim light to bright light (ex: your subject moves from indoors to outdoors), and avoiding a blown out second half of your shot means knowing how to "ride the iris."
So, these four videos should get you nice and acquainted with the basics of shooting with ND filters. If you're a professional/very experienced with ND filters, feel free to share some advice, tips, and tricks down in the comments for those who are new to the whole filter thing!