Smart Lock was introduced in Android 5.0 Lollipop. It was derived from Motorola's feature which debuted in the original Moto X in 2013. Basically, when your device is connected to a Bluetooth device or an NFC device you can have it set to stay unlocked as long as it's connected. What we mean by this, is if you have a PIN or password on your device, you won't need to use it as long as it's connected to a Trusted Device. Which for a lot of us would be our Android Wear smartwatch, or maybe a Bluetooth speaker. Or even our car, if our car has Bluetooth. It's a pretty useful feature, and Google has been rolling out updates to it ever since it came out in Lollipop.
The newest update notifies you when Smart Lock locks your phone after 4 hours of inactivity. Which is actually a good thing. So that if you haven't used your phone for 4 hours, and your Trusted Device has been connected the entire time, it'll let you know that the device has been locked again. Which increases security.
This is all part of the new Google Play Services update, which brought it to v7.5. Which started rolling out yesterday and you should have the newest version within a few weeks. These types of updates always take a bit to roll out to everyone, and that's to be sure there are no show stopping bugs.
Just recently, Google added On-Body Detection and Trusted Voice to Smart Lock. So as long as your device is on you, the device would remain unlocked. However, if it's set down on a table or in a chair, it'll be locked again. Again that's to improve security and get more people to use a PIN or password on their device, so that their data is protected on their smartphone.
Smart Lock is part of Android 5.x so you will need to be running Lollipop on your device to see this new update. It's unclear whether you need Android 5.1 though, it should work with just Android 5.0 and the newest version of Google Play Services.