hi can anyone explain to me what "withholding tax" is

Thank you

Google is your friend:

http://en.wikipedia.org/wiki/Withholding_tax

The way I understand it, it is just the money that gets taken out of your paycheck (or any income such as interest) automatically.

I have always had a beef with it too. Who knows what various entities do with your withholding tax when it is in their hands, or in the interim between when it is in their hands and then to the govt. I think there are many withholding tax scams in Thailand.