What is asset server?

Unity asked me to install something called SourceGear DiffMerge, which I did, and… I don’t really know what it is and how to use it?

Can someone explain it to me? Btw, I’m still a beginner at these things…

The asset server is a software to centralize backup and version control of your Unity project.

http://unity3d.com/support/documentation/Manual/Asset%20Server