Data doesn't always come in the format that is very useful or helpful for performing analysis and sometimes can be re-organized to make things easier to view. Reshaping can change the relative position of certain information to help draw emphasis between different pieces of data. Consider the following example:

                     A         B
first second                    
bar   one     0.721555 -0.706771
      two    -1.039575  0.271860
baz   one    -0.424972  0.567020
      two     0.276232 -1.087401

Using the .stack() method the data can be modified to look like this

first  second   
bar    one     A   -0.727965
               B   -0.589346
       two     A    0.339969
               B   -0.693205
baz    one     A   -0.339355
               B    0.593616
       two     A    0.884345
               B    1.591431

You can also include an argument when using the stack method to specify which columns you want to stack. The same logic and usage applies when using the inverse function .unstack().

Both .stack() and .unstack() will return a sorted copy of the dataframe.