In Python, whitespace is used to structure code. Whitespace is important, so you have to be careful with how you use it.
The code on the right is badly formatted.
def spam():
eggs = 12
return eggs
print spam()
You should see an error message like this one:
File "<stdin>", line 2
eggs = 12
^
IndentationError: expected an indented block
Now let's examine. You'll get this error whenever your whitespace is off.
To correct this you should properly indent the code with four spaces before eggs on line 2 and another four before return on line 3.
You should indent your code with four spaces.
>>> def spam():
... eggs = 12
... return eggs
...
>>> print spam()
12