It works like this: Take a smaller number such as 10!. I hope you know what 10!(ten factorial) means. It means this: 10 x 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 =3,628,800. Notice that 2, 3, 4, 5, 6, 7, 8, 9, 10 are ALL factors of their product of 3,628,800. This means that 3,628,800 will divide any one these factors EVENLY. So, for example, 3,628,800 / 7 =518,400. OK?. Now, if you add another 7 to 10!, you will have:3,628,800 + 7 =3,628,807. Now, if you divide this new number by 7, you will get the following:
3,628,807 / 7 =518,401.... So, now you can see 10!+7 will divide 7 EVENLY again, and the original quotient of 518,400 is gone up to 518,401. And the 1 at the end of 518,401 simply means we have 1 extra 7 because we deliberately added to 10!. And this applies to all numbers from 2 to 10 when added to 10!. I hope this is clearer now.