Answer 1

it's a variable name. you could name it whatever you like.

Answer 2

If you change the name of a variable in a script, you need to use one not still in use, and to change ALL references to it in your your script ^^
Suppose you have something like:

num = 7
var = 3
user = int(input())
num = num * user * var
if num == 42:
print("good answer")
else:
print("you lost")

... you must replace 4 occurrences of the word 'num' by anything you want but not 'var' and 'user' which ared already identifier for some value ;)