Tell me about this nikkor lens?

I'm a beginner at photography. I have a Nikon d5100 & 2 lenses. I have an 18-55mm and a 55-300mm I was offered to buy a nikkor 50mm f/1.8 ai

I have absolutely no idea about what kid of lens this is. Is it an upgrade or down grade from my lenses I already own? What is the difference?

Here is a nice reference http://www.kenrockwell.com/...0f18ai.htm

The ai is, I believe, old technology. What is the "speed" of your 18-55? If it is say 4.5, this old lens could capture much more light. In theory it could also yield sharper images due to having fewer elements for the light to pass through. You don't need it for focal length as your smaller one has that covered.
Your camera, however, is digital and auto-focus. The digital circuitry doesn't care what the lens is, but if you rely on AF, the old lens won't necessarily work with that.
If you can get it for cheap, it might be a nice addition for when you are going to be doing, say, only portrait work at a fixed distance.

It's a different kind of lens, for one thing. It's a prime lens, meaning that it doesn't zoom in and out like your other two lenses. And it's probably not a bad lens at all; it will let you shoot in lower light. But - and there's a BIG but - you'll have to find out with certainty is whether or not is has something called a "focus motor" inside it. It looks like quite an old lens, so it's quite possible that it does not.

Put in simple terms, a focus motor is the thing which lets a camera focus a lens using autofocus. To use autofocus, either the lens, the camera, or both, needs to have this motor.

Your camera - the D5100 - does not have a focus motor (a way to autofocus lenses), so if the 50mm 1.8 lens doesn't either, then you will NOT be able to autofocus that lens on your camera. You'll only be able to focus it manually. If this is not a problem, then it might well be a decent lens for your camera, as long as the price is reasonable. However, if you tend to use autofocus a lot, and if the lens does not have a motor, then you won't want it.

Check with the seller first; if it's possible, try the lens on the camera and see if you can get it to autofocus.

As a beginner, you have enough lenses already.

Learn the fundamentals of photography with what you have.

Spend any extra money you have on a faster computer, larger hard drive and maybe a copy of Adobe Lightroom 5.

That's an old manual lens, and definitely won't autofocus with D5100. You could use it manually but you'd be better off purchasing a 50mm lens that is properly compatible with your camera. A 50mm prime is a good lens to have in your arsenal, especially for low light or shallow depth of field work.

It's a very old manual lens, a 50mm prime f/1.8 lens - it won't autofocus, and you won't be able to use it in any automatic exposure modes either, you can only use it in manual mode and you would also need to use manual focus.

Just use the lenses you have already. If you don't know what a 50mm prime lens is or how it differs from the lenses you own already, then I can almost guarantee that you won't need one. I agree with fhotocace - you need to learn more about your camera before you think about buying new lenses.