Can you explain me the use of variable k in following program

First of all, there is a bug, try with input 5 or 7.

Quote:

i want to know why we use k in this code.

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

27.3. pdb — The Python Debugger — Python 3.6.1 documentation[^]
Debugging in Python | Python Conquers The Universe[^]
pdb – Interactive Debugger - Python Module of the Week[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

Even if properly indented (thanks to Griff), your code is broken. Try instead

num = int(input("enter a number "))
i = 2
prime = True

while num > i:
  if num % i == 0:
    prime = False
    break
  i += 1

if prime == True:
  print("number is a prime number")
else:
  print("Number is not a prime number")

Please note, this is a particular inefficient way to establish if a number is prime.


[Update]
As 0x01AA gently :-) suggests, you may also write

if prime:
  print("number is a prime number")
else:
  print("Number is not a prime number")

[/Update]