Where clause in querybuilder and eloquent

I've seen some code like this one below:

public function scopeUserId($query, $user_id)
{
   return $query->whereUserId($user_id);
}

Now, the question is what is whereUserId pertain(though it is verbose enough, but the syntax gets me confused) and where can I find it in the laravel documentation?

via Chebli Mohamed