Sometime in the near future you may need to install the Microsoft .NET v3.0 Framework in order to run a particular program. The Microsoft .NET Framework is a set of predefined functions that applications can call that are able to preform advanced operations. If you're a programmer the .NET Framework can help you produce your applications quicker, and even help make them more robust.
If you download a program that requires you to install the Microsoft .NET v3.0 Framework, then it will not run until you install this software on your computer.
Here is the latest version of the .NET Framework that you can download. If you want more information on this software then check out this site.