What is an Auth Code (also called Authorization, Security Code Code or Epp Code)?

What is an Auth Code (also called Authorization Code, Security Code or Epp Code)?
For Domain Transfer, the registrar is asking me for a code. Where do I get this?

The Auth-Info Code is a unique code generated on a per-domain basis and is used for authorization or confirmation of a transfer request. Some registrars offer facilities for you to generate and manage your own Auth Info code. In other cases, you will need to contact the registrar directly to obtain it. The registrar must provide you with the Auth-Info code within 5 calendar days of your request.
Domains ending with .com, .net, .org, .info, .biz, .us, etc. require authorization codes for domain transfers.