Nikon d3100 and Nikon 50mm f/1.8 D AF Lens?

I'm getting a Nikon d3100 (which does not have a built in Auto Focus Motor) and want to get a Nikon 50mm f/1.8 D AF Lens with it.
People say that it won't autofocus with it.
What does this mean? Does it mean it won't autofocus videos or pictures as well? Could you please explain this to me?
Thanks for any help.

Added (1). Thanks, but could you explain further what it means by 'won't autofocus'? I'm new to SLR's.

It means the 50mm lens will have to be manually focused on both pictures and videos. You will have to turn the focus ring on the lens and determine for yourself, through the optical viewfinder, if the image is in focus or not. That's basically how we did it eons ago with mechanical film SLRs.

The 50mm f/1.8D AF doesn't have a built-in motor. Therefore the SLR needs to engage the rotors inside the lens to perform the auto focus. But the d3100 doesn't have a motor to do this. Therefore auto focus won't work. But you can focus the lens manually. The light measuring and focus indicator will however still work.

edit: In order to auto-focus, the SLR must turn the focus ring on the lens. On the 50mm 1.8D, it is done physically by a rotating pin on the camera that connects to the lens. So the camera will rotate this pin to move the focus ring until the lens has reached the optimal focus point. But the D3100 does not have this pin, therefore it can't control the focus ring. On the DX lenses, that have built-in motor, the SLR just instructs the motor in the lens to turn by electric connections on the lens-mount.

Only Nikon AF-S lenses will auto focus on your D3100. Nikon chose to not put a focusing motor in the D3000, D3100, D5000 and D5100 so you'd have to buy the more expensive AF-S lenses that have a focusing motor in the lens. The Nikon AF 50mm f1.8D is about $140.00 while the new Nikon AF-S 50mm f1.8G is about $220.00 - a nice extra $80.00 or thereabouts in profit for Nikon.

If you use the AF 50mm f1.8D on your D3100 you will have to manually focus the lens. It will not auto focus on your camera.

Autofocus is a beautiful function on modern DSLR's basically, it focuses the picture for you. There really isn't much more to explain. It will not autofocus on your camera, meaning, you have to focus the camera manually by twisting a ring on the lens until the picture is as sharp as you can get it.

as for what WILL autofocus, look at the Nikon 50mm f/1.8G lens- that has an autofocus motor in the lens and will autofocus on your camera

'Won't autofocus' means that it will not 'lock on' focus automatically. You have to turn a focus ring on the lens until the image in the viewfinder comes into focus.

This was actually what we had to do with lenses back in the days of 35mm SLR's - it's not hard, just takes a bit of getting used to in these days of 'auto' everything.