What is 'AdBlock'?
AdBlock is content filtering and ad blocking extension which can be installed in the browser. AdBlock allows users to prevent page elements, such as advertisements, from being displayed. As of July 2014, AdBlock is the most popular Google Chrome extension, with over 30,000,000 users, and the most popular Safari extension. (From Wikipedia)
How AdBlock affects my site?
As explained above, AdBlock designed to block and remove advertisements. In most sites, the biggest (and sometimes, the only) revenue comes from advertisements.
For that reason you don't want your visitors to use AdBlock. It's directly affects your income.
More visitors exposed to advertisements = potential to earn more money.
What 'AdBlock Detector' does?
This add-on will detect if visitor's browser is running an AdBlock.
If so, it will popup an alert and ask (or force) to disable it.
You can use it to make people aware of the consequences to the site of using this extension, and nicely ask to disable it.
Or you can force to disable it, and not allow to enter into the site until then.