source code vs binary code
Computer programs are written in source text format, and are converted afterwards into another code, that computer can read and understand. This translation is made thanks to a program called compiler. The problem is, that the indications to run the program, when wirtten in a language that the computer reads, are made of 0 and 1 (that is in binary code) and are therefore no longer understandable for us. To understand the functionning of a program, you need access to the source code.