In light of the Stagefright vulnerability that rocked the Android world a few months back, Google has promised to push out security updates for Android on a monthly basis – starting of course with the Nexus line of devices. That monthly updated is rolling out OTA as of time of writing, first to Nexus devices, then to AOSP builds.
The Stagefright vulnerability is basically an opening to hackers who can take advantage of your device when you try to play a video sent via MMS/SMS or on your browser. The file downloaded will be a malicious one, and the hackers can then take control of your device via that single file.
The security update will be for devices with builds LMY48X or later, or a device running android Marshmallow. Google also said that they will publish links for updated AOSP builds when they become available.
Hopefully we can start being secure in our Android devices again, although there will still be hackers out there with malicious intent. Watch out for another update from Google after a month or so.