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 that mean? 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." Sometimes this means that something that isn't an ad is removed. An image doesn't appear when it should, for example, or, if the word "ad" is part of the site's name, the entire site is blocked! In cases like this, the filter list authors will usually add an exception for the site, whitelisting improperly blocked resources so the site works the way it should.
A good example of this is the National Advertising Initiative website, networkadvertising.org. 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. Note that this site doesn't have third-party ads. Whitelisting it doesn't violate EasyList's mandate to block ads everywhere it can.
Can I enable AdBlock on the site anyway?
The only way not to have the site whitelisted is to unsubscribe from the filter list. But again, there usually won't be a need to run AdBlock on the site anyway. The list authors generally will not whitelist sites that use third-party ads, even if the list breaks the site.