Sampler
I don't own it so I can't tell you much about it but I can tell you what it is. It's a software sampler, just like a hardware sampler that you might buy. In case you aren't sure what that is, basically a sampler is something that you would use to play *samples*.
Say you want to add some guitar to a song, but you don't play guitar or have a guitar player that can play it for you, you can use a sampler to add a guitar part. In a nutshell a *sample* is a recording of a sound, in this case a guitar. It actually gets much deeper than that but for now that should be good enough for you to get what I'm saying. You load the sample into a sampler (which is what gigsampler is) then you can *play* the guitar sounds from any midi instrument, most likely a keyboard.
You actually have a pretty good piece of software there, Gigasampler isn't cheap, especially if it's the full version and not the Lite version. Many pros claim that it's the best sounding sampler out there. One problem with it is you have to have it on it's own computer, seperate from any other music software you might be running, because it takes so much memory and system resources.
Hope that helped.