HTTP has nothing to do with Java.
HTTP stands for HyperText Transfer Protocol. It is a communication protocol built on top of TCP/IP. Status 200 (if you had read the links, this would all have been explained) means that the HTTP request has succeeded, and information has been sent back to the client (requester), if appropriate.
>>> I mean that I need to send a login request to my client side because my client want to make sure that my site here is a valid vendor. I don't know how to start with the coding. Any idea of it?
Do you mean that your "client" has a HTTP server running (such as apache or IIS), and you need to make a HTTP request to them (ie as a browser would to a web server) ?
The best advice I can give you is to better understand what is expected of you, and also to understand what is involved with HTTP ....
Good luck !