Whitespace is important in Python. Actually, whitespace at the beginning of the line is important. This is called indentation. Leading whitespace (spaces and tabs) at the beginning of the logical line is used to determine the indentation level of the logical line, which in turn is used to determine the grouping of statements.
This means that statements which go together must have the same indentation. Each such set of statements is called a block. We will see examples of how blocks are important in later chapters.
One example of how wrong indentation can give rise to errors is
i = 5 print 'Value is', i # Notice a single space at the start of the line. # This is an error! print 'I repeat, the value is', i
When you run this, you get the following error:
File "<stdin>", line 2 print i ^ SyntaxError: invalid syntax
Notice that there is a single space at the beginning of the second line. The error indicated by Python tells us that the syntax of the program is invalid i.e. the program was not properly written. This means that you cannot arbitrarily start new blocks of statements (except for the main block which you have been using all along, of course). Cases where you can use new blocks will be detailed in later chapters such as the control flow chapter.
Do not use a mixture of tabs and spaces for the indentation as it is not cross-platform compatible. I strongly recommend that you use a single tab or two or four spaces for each indentation level. Choose one of these three indentation styles and use it consistently i.e. use that indentation type only.