How do I use `global` variable in Python?

Recently, I have learned the concept of a global variable in Python. I am not aware that how can I use it in my code. I have found a code that contains the global variable and I’m not getting how the global variable is created and how it works in the code that is below:

Can someone explain to me how I can use the global variable in my functions? If I want to pass a variable of one function into another function. Am I able to do this using the global variable?