what does 'if x.strip( )' mean?

The method strip() returns a copy of the string in which all chars have been stripped from the beginning and the end of the string (default whitespace characters).

So, it trims whitespace from begining and end of a string if no input char is specified. At this point, it just controls whether string x is empty or not without considering spaces because an empty string is interpreted as false in python

In Python, "empty" objects --- empty list, empty dict, and, as in this case, empty string --- are considered false in a boolean context (like if). Any string that is not empty will be considered true. strip returns the string after stripping whitespace. If the string contains only whitespace, then strip() will strip everything away and return the empty string. So if strip() means "if the result of strip() is not an empty string" --- that is, if the string contains something besides whitespace.