I would like to purchase a Nikkor telephoto lense but what does all this mean?

AF-S Nikkor 300mm f/4D IF-ED?
I looked up the specs on Nikon's website but i have no idea what they all mean or how they work… Is anyone familiar with they mean?

Added (1). oh, and I have a 70-300 mm telephoto lens but its an older version, the kind that extends manually. I picked this one because its compatable with both my D50 and my N75.

AF-S = Auto Focus with a special "Silent Wave Motor" built inside the lens for faster and silent auto focusing

Nikkor = brand/manufacturer

300 mm = focal length

f/4 = biggest aperture (f/4 is good for a 300 mm lens, it means you still can make pictures in low light situations with fast shutter speeds where you can't use flash light. But it also means the front lens element is relatively big, therefore heavy, because the lens has to gather a lot of light)

D= Distance. (Information about distance is transmitted to the camera for 3-d exposure with flash and/or matrix metering)

IF = internal focus. (The glass parts move inside the lens while focusing. The front lens element doesn't move and the lens keeps same length during focusing. This offers better and faster focusing. The center of the weight of the lens changes less so it is more stable which is important for tele lenses. It reduces possible small vibrations and movements which could result in unsharpness)

ED = Extra low Dispersion (reference to the type of glass and coating being used to make it optically a better lens. You can recognize some Nikkor ED lenses by the golden ring around the front)

This lens is an expensive lens because it has optically a very high quality, a big aperture compared to it's focal lenght and very good and fast focusing capacities.
It's designed for high demanding tele photo photography, for example in sports where you need fast shutter speeds and fast focusing or nature photography.
It's seen as a long tele lens, your D50 will even make a 480 mm lens (compared to 35 mm cameras) out of it due to the 1.6 magnification of the focal length, a length you normally won't use very often.

The real question is: do you really need this expensive professional high performance lens (for the only reason it fits your two relatively cheap cameras), is your photography at a such level that it would contribute to your photography?
If yes: get it.
If no: search a little bit more and save a lot of money. There are cheaper 300 mm lenses. Though I'm not familiar with a Nikon N75 and Nikon D50 surely this is not the only lens which fits both your cameras. If a lens fits the Nikon N75 than it also fits your Nikon D50. Just don't get a DX Nikkor (doesn't fit the N75) or pre AI type Nikkor (doesn't fit the N75 and D50).
Be aware that many old Nikkor lenses still fit the modern Nikons but don't support all the automatic modes on your Nikons. See link at bottom for further information about this.
There are also other manufacturers like Tokina, Sigma and Vivitar who have good lenses but cheaper than Nikkor.
(By the way: AI = Automatic Indexing. That means your camera measure light by light meter coupling with the aperture fully open and still takes in account the aperture which is actually set. But AI doesn't work on both of your cameras, still can be used without light metering though.
DX lenses are made only for the digital Nikons because they are designed for their sensors which are smaller than 35 mm film size)

THis is not meant to be a "smart-assed response" but, it you really have to ask these questions, you would be better off, unless rich, to visit your nearest "Ritz Camera" store, and purchase a good, but less expensive lens. Nikkor is a very costly lens.