To "call" means to make a reference in your code to a function that is written elsewhere. This function "call" can be made to the standard Python library (stuff that comes installed with Python), third-party libraries (stuff other people wrote that you want to use), or your own code (stuff you wrote). For example:
#!/usr/env python
import os
def foo():
return "hello world"
print os.getlogin()
print foo()
I created a function called "foo" and called it later on with that print statement. I imported the standard "os" Python library then I called the "getlogin" function within that library.