The domain authorization code (also it can be referred to as an Auth Code or an EPP Code) is a secret code that is necessary if you want to transfer your domain name. It's like a safeguard against unauthorized domain transfers.
The thing with EPP codes is that every domain has a unique code which is assigned by the domain registrar, it also can be changed by them, so you can feel safe. Your domain will not be transferred without your permission!
Keep in mind that EPP key is required when transferring.com, .net, .org, .biz, .info and .us domain names. If your domain name is different, you will not need EPP code. Other security measures apply for different domains.