The order of the stack shows the flow of execution. printTwice was called by catTwice, and catTwice was called by __main__, which is a special name for the topmost function. When you create a variable outside of any function, it belongs to __main__.
Each parameter refers to the same value as its corresponding argument. So, part1 has the same value as chant1, part2 has the same value as chant2, and bruce has the same value as cat.
If an error occurs during a function call, Python prints the name of the function, and the name of the function that called it, and the name of the function that called that, all the way back to __main__.
For example, if we try to access cat from within printTwice, we get a NameError:
Traceback (innermost last):
File "test.py", line 13, in __main__
catTwice(chant1, chant2)
File "test.py", line 5, in catTwice
printTwice(cat)
File "test.py", line 9, in printTwice
print cat
NameError: cat
This list of functions is called a traceback. It tells you what program file the error occurred in, and what line, and what functions were executing at the time. It also shows the line of code that caused the error.
Notice the similarity between the traceback and the stack diagram. It's not a coincidence.