As you start to write larger programs, you will want to save the function de?nitions in multiple ?les, collected together according to what they do. So, for example, we may collect a set of utility functions together into in one file, called utility.py. This ?le is known as a module in Python.
Now, if we want to use those procedures in another shell or file, we will require to say import utility so that all those procedures become available to us and to the Python interpereter. Now, if we have a function in utility.py named foo, we may use it with the name utility.foo. You may study more about modules, and how to instance reference functions described in procedures, in the Python documentation.