Starting today a new software update is headed towards what is basically the entire Nexus line of mobile devices released within the past few years. We're talking about the LG Nexus 4, LG Nexus 5, Motorola Nexus 6, Asus Nexus 7, and HTC Nexus 9.
All of these devices are about to receive a new software build, yet the Android version will stay at 5.1.1 even after you apply the update. The new build number is LMY48M, and this should be identical for all of the aforementioned products except carrier versions of the Nexus 6. In this case there will be different build numbers - for example T-Mobile's is LYZ28K.
This seems to be the first of the recently promised monthly security updates. That commitment was revealed by Google after the big Stagefright vulnerability was discovered. Alongside the security patches, the new update should also come with some unnamed bug fixes.
Since there are no new features inside, these updates are small. We're talking around 30MB for the Nexus 6, and under 20MB for other devices. So it really shouldn't take you long to download and install. It might however be a while until you get that update notification, because as usual Google's opted for a staged rollout.