eBay is texting you the security code to the number you are trying to add/edit/whatever to your account that they are trying to verify. Obviously you don't know it until you get it, that's the point (so someone else without access to your phone can't use the number, e.g. someone picked it randomly or is trying to steal your identity).
Are you not getting the text? Is it a landline that doesn't get texts? What is the actual situation and what is your actual question?
