192.168.159.130 is the IP address of the server you want to crack and
http-get is the protocol which is used to send passwords, in this case plain HTTP authentication, and
/whmcs/admin/ the directory on the server. Webservers can have multiple sites and configure different users and passwords for them. So the accounts which are valid on
/whmcs/admin/ might be different than those on
That's why you need to tell hydra which directory exactly you want to crack. There is usually no way to tell how many directories with individual http password protection are configured on a server and what other functionality there is on the server which could be called an username+password authentication (like login forms).
Usually you wouldn't run a tool like
hydra blind. You would try different URL paths and different protocols on a webserver until you get prompted for a password, find out what technology is used to implement that password prompt, and then run hydra on it.