How do you read loops? (python)

What is the thought process when evaluating a loop because I really have no idea how the shell gets these answers. (a= 12, b= 2, c= 4, d= 6)

while A <= 10:

A += 2

if A%3 == 0:

B += 1

else:

C += 1

D += 1

Python newbie here