What is a factorial
You have 3 seats. Seat A, B and C. You need to find out how many ways you can arrange sitting order for your friends friend 1 through 3 given seats A, B and C. Let's say you choose a seat for a friend at a time. And you follow the order 1 through 3. Friend 1 has 3 seat choices. After you choose one of the 3, 2 are left. So friend 2 has 2 choices. Lastly, friend 3 has 1 choice.