Google's previously announced enhancement to the Verify Apps framework is rolling out to users now, according to the official Android blog. Your device already has the standard Verify Apps system built-in that scans at the time of installation, but the new version will be watching all the time for suspicious activity.
Verify Apps compares each app you install with known malware signatures, but there's always a possibility you are downloading a form of malware that hasn't been identified yet. An app might also install malicious code by some means after it has passed through the filter successfully. That's why Google is ramping up its scanning system. Constant monitoring ensures viruses and malware won't be able to hide on your phone or tablet.
Google says fewer than 0.18% of installs in 2013 triggered the malware scanner, so it's not too likely you're ever going to need the malware scanner, but it's nice to know it's there. The update should arrive via Google Play Services without any user interaction in the near future.