A 50MM lens is a portrait lens. It's one of the prime lenses to use in, for example, indoor situations photographing people.
Just remember, the smaller the focal length, the wider the angle. 50mm gives about the same relative field of view as does your eye.
When you place the camera in front of your eye as if to take a picture, and then lower the camera, you'll see that looking through a 50mm lens looks about like you see without the camera. Things are in about the same place.
Smaller number focal lengths, like 35 or 28 or 14 mm, all offer progressively wider fields of view. Larger numbers are for high magnification/long distance photography.
The two most common lenses used for portrait photography are the 35mm and 50mm prime lenses. You SHOULD have one or the other in your lens kit.
Remember when shopping for Nikon lenses, the focal length in mm is effectively different if you have a crop sensor camera and you're looking at a full frame lens. Or vice versa.
The 18-55mm kit lens for a crop sensor (DX) D7200 is the equivalent in terms of relative angular field of view and zoom range as a full frame (FX) 24-70mm lens. They do the same job, but one is FX and the other is DX and the FX version is much more expensive.
If you put an FX full frame lens on your DX camera then you will get LESS zoom, wider field of view. (Because the FX lens makes a larger image that is intended to fall on a larger sensor.)
If you put a DX crop frame lens on an FX camera then you will get MORE zoom, narrower field of view, because the DX lens makes a smaller image intended for a smaller sensor.)
The difference is about the ratio between 18 and 24, or 3/4. Or 1.333 looking at it the other way.
I recommend that you should eventually have the following lenses in your lens kit, to cover most applications.
18-55mm VRII for general everyday walkabout photography
55-200 or 55-300mm lens (varying price points) for medium zoom range photography
A prime lens, 85, 50, or 35mm for portraiture at different ranges
If you're into birding, aviation photography, sports events, or wildlife, you WILL want a big zoom lens. The Nikkor 200-500mm F/5.6 lens is very good and at about 1500 dollars it's not exactly cheap but it's actually something of a bargain. I have one and it just delivers the goods. (Yes, it's an FX lens but that just means you'll get free extra effective zoom range out of it. It'll behave as if it's a 266-670mm lens. )
You may want a macro lens for serious up close photography of bugs and flowers. If you do, the Nikkor 40mm lens may be just right.