In short, the ad review center, will give publishers a method of reviewing all placement targeted ads on their site and decide if they want to block those ads or not. AdWords advertisers can specify if they want to place an ad on a site that runs AdSense. Some AdSense publishers want control on which advertisers can display ads on their site in this manner. So AdSense is rolling out the "Ad Review Center" to address this concern.
First, you need to opt into the Ad Review Center. Once you opt in, then Google will show you all the placement-targeted ads currently targeted to your site. Then you can make an informed decision if you want those ads to continue to run or stop them by blocking them in the Ad Review Center.
When you block ads, Google asks you to provide a reason, which is then shared with the advertiser, so the advertiser can use this information to improve their ads. Google also uses this information internally to help improve their products.
Most AdSense publishers won't be able to see the Ad Review Center, Google plans on rolling it out over the course of a few months. You will know you have it when you see a "green notification box at the top of your 'Competitive Ad Filter' page." The competitive ad filter page is under the ad set up section.
Part of this is the option to auto-allow versus allow manually. As described in the AdSense help section:
When you first opt in to the Ad Review Center, your setting will be defaulted to auto-allow, which means that all new ads that are placement-targeted to your site will be allowed to run automatically. Even when ads are already appearing on your pages, you can review them and take action by visiting the Approved tab in the Ad Review Center later. You'll still be able to block individual ad groups or advertisers on this page if you choose.
The alternative to auto-allow is manual review. When you choose manual review, you'll have 24 hours to take action on new ads before they are automatically allowed to run on your sites.
Before you choose to manually review ads, keep in mind that doing so may have a negative revenue impact. Ads that are awaiting review won't compete in the auction on your site, and therefore won't drive up potential earnings on your site. Please consider the potential revenue impact before choosing manual review instead of auto-allow.
As you can imagine, AdSense publishers are delighted that this feature is coming their way.