Mirror Lockup
With a SLR (Single Lens Reflex) camera, whether film (SLR) or digital (DSLR), you are viewing the scene directly through the big lens mounted in front. To get from the lens to your eye, the light reflects off of a mirror, and then a pentaprism. I think the A100 uses another mirror instead of a pentaprism to cut cost though. The mirror is located behind the lens, and just in front of the shutter. When the shutter is released, this mirror flips up and out of the way long enough for the shutter to expose your spiffy Nikon CCD sensor, and then the shutter closes and the mirror flips back down to the viewing position.
All of this flipping, regardless of the manufacturers best efforts to quell it, causes a violent vibration. Mirror lock up forces the mirror to flip up and stay up, leaving only the shutter to determine exposure. This will result in sharper, crisper images but at the cost of your viewfinder, as the job of the mirror is to get light there.
Take a picture and notice when everything goes black for the exact amount of time as your shutter speed (maybe a littttle longer). This "blackout" is when your mirror is flipped up.