FX is Full Frame (same size as 35mm film format), and DX is Crop Frame....same as the extinct APS-C film format.....smaller. You can use most recent Nikon lenses on most recent Nikon cameras....FX lenses on FX, or DX cameras....DX lenses on DX, or FX cameras. However, while most FX lenses will work fine on DX cameras, the DX lenses on FX cameras will produce serious vignetting all around the image....an image surrounded by black. FX gear is bigger, and more expensive, but generally better, and more Professional / Prosumer oriented for serious photographers with a bit more money. DX gear generally costs less, and is slightly smaller, and lighter weight. It is more amateur oriented, and often produces lower quality images. FX gear usually works better in lower light. FX gear is what it is....a 50mm FX lens is a 50mm lens, and produces a 50mm Field Of View. A DX system produces a 1.5X crop factor, so a 50mm DX lens produces the Field Of View of a 75mm lens on an FX camera. Even if you have a DX camera now, buy FX lenses. When you upgrade to an FX camera, later, you will not have to replace your lenses. I used mainly FX lenses on my DX format D7200 camera. When I bought my FX format D750, I already had the FX lenses I needed for it.

BTW....Since you don't know the answer to this question, if you're buying a camera, go with a DX camera. I would suggest a D7200 (D7500 or even the more expensive D500 if you shoot sports a lot, otherwise the D7200 is an awesome camera), a Nikon 28-300mm lens and a Tamron 10-24mm (wide angle, the new version). With this kit you wouldn't need to buy another camera or lens.....except perhaps the 35mm or 50mm f1.8 for low light and to have a prime.

If you shoot a lot of sports my lens suggestion might be somewhat different. However, you can't go wrong with the 28-300mm.

When cameras went digital, camera makers had the ability to play with the sensor that takes the photo. In a DX formatted camera, the sensor is a bit smaller, causing the photo to actually be cropped in camera, making the photo slightly larger when viewing it. In Canon, it is a 1.6 crop, and in Nikon, it is a 1.5 crop. FX utilizes the full sensor size of 35mm film cameras. What this means to you, as a photographer, and as a beginner, is little to nothing. Both FX and DX cameras, on a general basis, take great quality photos. When it will make a difference is when you become more skilled and you expand your interests. A DX camera will do well in good light for wildlife, landscapes, and portraits. Actually, a lot of bird and other wildlife photographers prefer DX because of it's cropped sensor allowing them to use a smaller and lighter lens when going out into the wild to get their photos. If you venture into photography that restricts your light, usually a full frame (FX) camera will work better, as they will allow you to use a higher ISO to compensate the lower light situations. Whichever you choose, try to buy FX lenses. You may use an FX lens on a DX camera, but a DX lens on a full frame camera does not work well. Hopefully this information is understandable and helpful. Many people start out with a DX camera because of initial cost, and many stay with this format and are very happy with their choice. Personally, I have both, and for what I use them for, they both compliment each other.

It has already been said, FX and DX are terms used by Nikon for the size of their sensors. FX is pretty close to the size of a film negative for 35mm film cameras. Many photographers feel better using a camera that has a sensor that simulates what they used to have when using a film camera.
Both images are hard to tell apart when using the same lens with the same technical data for a subject. FX has been king when it comes to noise performance but the new DX sensors are doing pretty good also. DX has a "digital factor" by which the image is enlarged to 1.5 times its size and that is a godsend for wildlife photography. It is very important that you know, if you do not already, that each camera should use the lenses made for it. It makes no sense to buy a FX camera to use DX made lenses. FX lenses can be used with the DX format but understand that the full capacity of the lens cannot be used because the whole DX sensor is not covered with those lenses. Come to think of it, a DX lens covers the WHOLE DX sensor and still it is not called "full frame."
I think these are the most important concepts to keep in mind.

rmorrison1116 got it exactly right, as did some other replies. I come from the film era. For digital I shot APS Canon EOS cameras for some years before switching to a Canon full-frame EOS 6D, and I really wish I had switched a couple years earlier before I took hundreds of great shots of shorebirds. They're great, but they have a fair amount of noise that they wouldn't have had I been shooting full-frame, and printing them 48" wide reveals too much of it. In a nutshell, if you're serious about photography and are likely to go forward with it for a long while, and if you can afford it, buy a full frame camera. You won't regret it. If you do buy a DX camera and lenses to go with it, buy full-frame lenses so that you will be ready for full-frame when you realize you have to move up to it.