1. Instructions written by a programmer in C language is called program. Any program is stored in a file (for example- rectangle.c) called source file and the code stored in it is called source code.
2. Source code is in human readable form. For machine (i.e. computer) to understand, it should be converted into machine readable form called machine code/object code. This task is done by C compiler which first of all checks for any syntax error and if there is no syntax error then convert source code into object code and creates new object code file (for example- rectangle.obj) automatically. This is done by pressing Alt + F9 key.
   
3. To execute any object code, it needs to be converted into executable code which is stored in a separate file (for example- rectangle.exe). This is done by pressing Ctrl+ F9 key. A new executable file will be created automatically which gives output of the program.
   

This is shown with the help of Figure 1 below-

Figure 1: Source Code to Executable Code