source code vs binary code
Computer programs are written by the programmer in plain-text code, and are then converted afterwards into numeric computer code that computer can read and process. This conversion from human code to computer code is made by software called a compiler. The problem arises because the computer code consists of only numbers (binary 1s and 0s), and is virtually impossible for humans to read. To understand how a program works you need more than just the compiled computer code; you also need access to the human-readable source code.