Postini describes themselves as:
"..the global leader in on-demand solutions that deliver on the promise of Communication Security and Compliance. With Postini, your company is protected against increasingly malicious internet-based attacks. Your compliance is ensured for government, industry, and internal mandates; and you can re-establish increased productivity for your users."
Google's reasoning for acquiring the company seems to lean towards more security for Google Apps, since they've received a "tremendous" response with "over 1,000 small businesses signing up for the service every day."
The Google blog even has a welcome post for the Postini team. The Enterprise Team at Google also has a bit more of an explanation on their blog too. And if that weren't enough, there's even an FAQ pdf document that provides even more detail.