Most software that you buy or download only comes in the compiled ready-to-run version. Compiled means that the actual program code that the developer created, known as the source code, has run through a special program called a compiler that translates the source code into a form that the computer can understand. It is extremely difficult to modify the compiled version of most applications and nearly impossible to see exactly how the developer created different parts of the program. Most commercial software manufacturers see this as an advantage that keeps other companies from copying their code and using it in a competing product. It also gives them control over the quality and features found in a particular product.
Open Source software is at the opposite end of the spectrum. The source code is included with the compiled version and modification or customization is actually encouraged. The software developers who support the open source concept believe that by allowing anyone who's interested to modify the source code, the application will be more useful and error-free over the long term.