How many 5-character strings can you make from the alphabet? Each position has 26 choices. Product rule: 26^5 = 11,881,376.
What if no character can repeat? First position: 26 choices. Second: 25 (one used). Third: 24. Fourth: 23. Fifth: 22. Total: 26 × 25 × 24 × 23 × 22 = 7,893,600.
Restrictions change the count per position. The product rule still works, but you adjust the numbers.