Source control is a mechanism that allows a development team to track individual changes of files in a project over time.
Have you ever used "Change Tracking" feature inside a Microsoft Word file? As you type, Word will track what you write, what you delete, and other assorted changes that you make. You are able to comment on each change, and later accept or reject them.
Source control works the same. As you work on a project that probably contains tens to hundreds of files, the source control system manages what is happening. You can audit the changes, and even rollback changes if they are not acceptable.
VS.NET Provides FREE version of TFS, Team Foundation Server.
Use it, feel it :)