What do you understand by stepwise refinement of the program?

The method of "Stepwise refinement" means to take an object and move it from a general perspective to a exact level of detail and this cannot be completed in one jump but in steps. The number of steps required to decompose an object into sufficient detail is in the end based on the inherent nature of the object.  "Stepwise refinement" of program shows a "divide and conquer" approach to design. In other words, break a complex program into smaller, more manageable modules that can be reviewed and inspected before moving to the next level of feature. Step-wise refinement is a powerful paradigm for making a complex program from a simple program by adding features incrementally.