They basically created sophisticated software to help publishers identify and sign the best advertisers for their web sites.
Here is a cute video from AdMeld on how it works:
Webmasters are trying to figure out why Google would spend so much on a company like this. A WebmasterWorld thread considers the talent of the company or maybe the technology something more valuable than what most of us see.
Forum discussion at WebmasterWorld.
Update: Google officially announced the deal on their blog a couple hours after posting this. Google said:
By combining Admeld’s services, expertise and technology with Google’s offerings, we’re investing in what we hope will be an improved era of flexible ad management tools for major publishers. Together with Admeld, we hope to make display advertising simpler, more efficient and more valuable, provide improved support and services, and enable publishers to make more informed decisions across all their ad space. These are all things our publisher partners have been asking us to further invest in. Of course, Admeld will continue to support other ad networks, demand side platforms, exchanges and ad servers, to yield the best possible results for publishers.