There have been a good handful of security issues with Android over the past few months (some of them more severe then others) so many manufacturers issued verbal commitments to keep their devices as secure as possible with monthly security updates. Google, as you’d expect, has been leading the charge (well before the fear mongering began that really escalated these issues) and they are ready to go with their first security update of 2016.
In total, the update patches up 12 different security issues, 5 of which they deemed as critical. The most extreme of these issues could have allowed “remote code execution on an affected device through multiple methods such as email, web browsing, and MMS when processing media files.” Thankfully all is patched up and the updates are rolling out.
My Nexus 6P just got the update today so I’m sure other Nexus’ should be getting it soon enough. If you haven’t already received a notification, you can always jump into your settings and do a manual check.