Just in case I didn't write that clear; the answer is Thousands Period.
Each digit in a number has a place value that tells you how much that digit is worth.
Each place value is ten times as large as the next place value to the right.
Numbers are arranged into three place values called periods
The places in the period repeat (hundreds, tens, ones.)
Commas are used to separate periods.
Place Value Chart
I have the same problem too helpI don't get it