Here's a scenario: You are visiting a site and notice that the AdBlock button in the browser toolbar has changed to a green "thumbs-up," indicating ads are not being blocked. You know that you have not told AdBlock to stop blocking ads on this site. You click the AdBlock button to see whether you may have paused AdBlock and forgot to resume ad blocking. When you select Enable AdBlock on this page, you see the following notice: "Sorry, AdBlock is disabled on this page by one of your filter lists."
What does this mean? It means that one of the filter lists you are subscribed to has disabled ad blocking on (or "whitelisted") that site.
Why would an ad-blocking filter list whitelist a site?
One of the ways filter lists remove ads is by blocking and hiding resources (images, scripts, and so on) with names that contain the word "ad." If the word "ad" is part of the site's name, however, the entire site could be blocked even it the site doesn't serve ads at all.
In cases like this, the filter list authors will usually add an exception for the site. That forces the filters to ignore resources that would otherwise be blocked, so the site works the way it should.
A good example of this is the National Advertising Initiative website, networkadvertising.org. This site doesn't have third-party ads, but every resource that makes up the site contains the word "ad" in its name. If EasyList didn't whitelist it, the site would be unusable.
What should I do?
You don't need to do anything. There is no need for AdBlock to run on the site, because it doesn't serve third-party ads. The list authors don't whitelist sites that use third-party ads, even if the list breaks the site.