The Publisher Center gives publishers a way to validate and update the data found within Google Ad Planner. Google Ad Planner shows advertisers statistics on traffic, demographics, and so on about specific sites on the internet. Ad Planners purpose is to give advertisers more information on sites that run AdSense for them to make informed decisions about their ads. The Publisher Center allows publishers to confirm the data Ad Planner displays, and also add more detailed information about their sites to be displayed in Ad Planner.
I have posted detailed screen shots of Ad Planner at the Search Engine Roundtable. Google has also posted a short video demonstration of Publisher Center: