A very quick, very brief overview:
CVS is a management system that keeps track of the changes to the source code files, which are in plaintext(same as .txt) format. It tracks the changes to each individual file, so if something goes wrong somewhere, an older version can be retrieved and the files compared to help find out how to fix the problem.
Each source code file contains a series of instructions that tell the computer what to do. There are two main types of files: .h and .cpp. ".h" is for header files, which are typically referenced by one or more files. They are included in the .cpp files, which contain most of the instructions for the computer.
The Freespace source code is written in 'C++', which is a new version of 'C'. C/C++ are compiled languages, which means you write the code, then you compile it with another program to turn it into an .exe. There are two big compilers: gcc and Visual C++ 6's. If you want to compile Fred, you'll need Visual C++. Otherwise, you can use either.
As for reading materials, there's another thread in the Hard Light forum with several suggestions.
As for where you can download the source code, go to
http://fs2source.warpcore.org/ and download the preconfigured batch file. Extract the zip in the directory where you want the source code, then open up a dos window, go to the directory where you extracted the zip file and type "startcvs -download fs2_open". Once the batch file has finished running, a directory called "fs2_open" should appear.
