What is a Domain Transfer Secret Used for, while registering .BIZ, .INFO, .US Domain Names?

The Domain Transfer Secret is a pass-phrase used at the time of transferring a domain name from one Registrar to another.

Suppose, you own a Domain Name abcd.biz at Registrar A. Now if you want to Transfer this domain name to another Registrar, say Registrar B. Then when you fill the form for transfer of abcd.biz on Registrar B's website, you will need to feed in this Domain Transfer Secret alongwith it.