Take a multi-digit number. Add up all of the digits. Add up the digits in the sum. Keep adding like this until you arrive at a single digit. Is there a name for these numbers?
The procedure is usually called “casting out nines.” A slightly faster method is to add the digits (which gives 46 in your example) and then take the remainder when that’s divided by 9. So 1 is the result.
Where I grew up, ‘casting out the nines’ was commonly taught as a way to check for errors in arithmetic (which admittedly wouldn’t catch errors of misplacing digits [e.g., writing 123 as 132).
For those of you following at home, this is saying that casting out nines can prove the presence of certain errors. It doesn’t say that casting out nines can prove the absence of all errors, because it can’t. Transpose two digits and casting out nines gives the same result.