A nested loop is a loop inside another loop. The inner loop runs completely for each single iteration of the outer loop. Example: for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) { cout << i << j << " "; } } prints 00, 01, 10, 11, 20, 21.
The outer loop runs 3 times. For each outer iteration, the inner loop runs 2 times completely. Total iterations: 3 times 2 equals 6 inner loop executions.