What is the difference and the function between "Global" and "Local" in python?

I would ask
What are the difference and the function between Global and Local in python? I really confused about this question.
Example:
def x():
global y
y = "Great!"

x()

print("He is" + y)

Someone, please help me answer this question and I'll give you some of this chocolate.🍫
Thank you!! ☺️