what is __name__ == __main__ in python code example

Example 1: if __name__ == '__main__'

if __name__ == "__main__":

Example 2: What does if __name_=="_main__": do?

# Suppose this is foo.py.

print("before import")
import math

print("before functionA")
def functionA():
    print("Function A")

print("before functionB")
def functionB():
    print("Function B {}".format(math.sqrt(100)))

print("before __name__ guard")
if __name__ == '__main__':
    functionA()
    functionB()
print("after __name__ guard")