INT takes a number and rounds it down. So if A1 was 15 then 15/12=1.25 rounded down would be 1.
MOD returns the remainder after a number is divided by a divisor (in this case, 12).
This is long division which I am sure most people have forgotten once you reach a certain age! Although understanding how it works isn’t completely neccessary it doesn’t hurt to. See this link for an explanation.
You should now be connected to your Exchange Online account. Enter the following to import your Contacts CSV file. The below code makes the assumption that your first is in the C:\ drive and is called contacts.csv. Rename as appropriate.