current_userid is set when you are viewing a users profile – so it is the user id of the member currently being viewed. If you are not viewing a user profile, it is 0.
loggedin_userid is the user id of the user logged in.
bp_is_home() does a if ( current_userid == loggedin_userid ) return true; else return false;
So bp_is_home() will return true if you are viewing your own profile.