They've done well explaining it. Just for your reference
Gigastudio is a software sampler. You can buy many instruments that are in giga format, such as pianos, strings, guitars, drums, etc. For instance, Virtuoso solo strings is 8 gigs of high quality solo violin, viola, cello, and bass samples. A sample is a recording of something. Take a gigapiano for instance. each of the 88 keys will be sampled at a number of different velocities. These samples are then mapped into the gigastudio format, and will be played as you send midi from your controller to your PC.

Some of the other posts might give you a better way to start out though. Gigastudio is for the person who needs the highest quality sounds for their musical productions. You need a heck of a lot of computer horsepower when you start using giga instrumets.