It means the web site's server is denying you access.
It could be that you're trying to get to a page that isn't configured to be publicly visible, or it could be that your IP address is being blocked.
Contact the web site owner, and explain what's happening. Sometimes, my readers get blocked from accessing my sites because their IP address has been blocked by my hosting company, for suspicious activity in the past. It usually just takes a few minutes for them to check the address, and unblock it.