From P2P Foundation

"Source Code - is the human readable form of computer code rather than just the machine-readable binaries, consisting of nothing than one and zeros. Source code needs to be compiled in order to run on a machine. Vendors of proprietary software do not give out the source code so that the functions of a programme can neither be checked nor changed."

More Information

In Free Software or Open Source Software, the source code should be freely available for modification.