Its a tool from Adsense which lets you block your competitor’s ads to show up on your site. It also helps you to block any unwanted sites like MFAs. The only con is that there is a limit of 200 URLs and believe me, that is not even close to sufficient.
Not only that, a wise use of it can help you to earn more from the same number of clicks by just blocking the low paying sites. The main problem is that you have to create the list of the sites to be blocked by yourself, there are no reliable automated process for this.
How to find low paying sites:
Theres no easy option available as I said before. You have to do them manually. You can use the Adsense Preview Tool to find out the ads appearing on your site and know the URL of that ads. This is helpful to avoid accidental click on your site. It works only with Internet Explorer 6.0 or higher. The instructions can be found here.
How to use the Filter:
Competitive Ad Filter To use this feature, you must be logged in to your Adsense Account. Click the AdSense Setup tab. A row of links will appear beneath the tabs - click Competitive Ad Filter. This page will display the URLs currently being filtered for your account. If this is the first time you’ve used your filter, the text box will be empty. Now you can paste the URLs to be blocked the the space and the hit Save Changes and it is all set. It may take up to 4 hours for the filter to work.