So spidered and indexed means the data can be found. Most of us now look for information by using a search engine on the Internet, whether that's Yahoo! or Google or Microsoft Bing. We use one of these to go and find information.
There's a reason why those search engines know where all the information is: they do what we call “spidering”. They have little things that kind of float around the Internet and they go and find where the information is so they can then point you to it when you look for a relevant term.
If you happen to have data that you say you're willing to share with the world, but you have it on a server that's not connected to the Internet or you don't allow Google or Microsoft or Yahoo! to spider and index, in my opinion you don't have open data. If it's open but nobody can find it, it's really not that open. So that's the first rule.