Understanding the If block

To print prime numbers between 1 and 20 I used the following code. While I understand the output was wrong because of the indentation, but I specifically want to understand what is happening here. Can please someone help me? Here is my code:

for n in range(1,20):
    flag = True
    for i in range(2,n):
        if n % i == 0:
            flag = False
            break
        if flag :
            print (n)