A neutral density filter is a screw on attachment for your lens that blocks some portion of the incoming light without (hopefully) affecting the light in other ways. It’s kind of like having a pair of sunglasses for your camera. ND filters come in different strengths (according to the amount of light they let through), different sizes (depending on the lens you’re using), and different glass qualities (just like how some lenses are better than others.)
What do you do with an ND filter?
Primarily, the ND filter is used to increase the shutter speed without increasing your F-stop and/or overexposing the shot. Common examples are waterfalls with the cotton candy water look and shots with the misty looking waves around ocean rocks and piers. With some experimentation you’ll find more creative uses like taking a picture of busy places without having the people/traffic show up or shots of someone looking sharp in the midst of a sea of blurry people.
Which one should you get?
These things aren’t cheap so you’ll want to give some thought to the type of shooting you intend to do. I wanted to do very long exposures during the day using my 24-70mm so I went with a B&W 3.0 ND Filter. The 3.0 is a 10 stop filter, or said another way, lets in just .1% of the light (so it’s more like a welding mask instead of sunglasses.) With that filter I can easily get 5-10 minute exposures in daylight. The downside is that you can’t see anything through your viewfinder with the filter attached so I have to set up, compose the shot, and then put the filter on trying to be very careful not to mess up the framing/focus.
Note: There are some variable ND filters out there where you can rotate them to make them lighter/darker (two circular polarizers screwed together?), but I’ve not seen them in action and don’t know about their quality.