A loop's block can contain anything, including another loop. The inner loop runs all the way through for each iteration of the outer loop:

for row in range(1, 4):
    for col in range(1, 5):
        print(f"({row},{col})", end=" ")
    print()

The outer loop runs 3 times, and each time the inner loop runs 4 times, so the inner block runs 12 times altogether. The print() after the inner loop (indented once, not twice) ends each row.

A clock is a good way to picture it: the minute hand goes all the way round for every single step of the hour hand.