I am creating Stack Overflow client application and have reach limit to request. So I decided to get the Key from my application, but got a little confused.

1. When I requested the Key I provided application site. What is purpose of this site ? Can it be just my home page, git account or a personal blog ? Or do I need to implement some functionality on this site ?
2. I got a Client Id, Client Secret and Key. If I understand correctly I need to use only key ? In which cases do I need the Client Secret ?
3. Do I need to do something after I get the Key to make it works ? I am using Rubyoverflow library, but I got error message when I set my key ("Invalid key")