Android Lollipop has recieved its first proper incremental update of this year today. The Android 5.1 update brings a bunch of bug fixes and a number of new features. The new update is rolling out to all devices running stock Android Lollipop.
The Android 5.1 update brings support for multiple SIM. The feature will now be baked into the OS itself, instead of OEMs developing their own implementations. Another major feature that has been added is the new Device Protection. This will enable your stolen/lost Android device to remain locked until you sign in with Google account password. The device will remain locked even if someone resets your device to factory settings.
After the new update compatible devices like the Nexus 6 will support HD voice. Currently HD voice is supported only on selected telecom operators, hence all Nexus 6 owners may not be able to observe the difference. Devices with the new update will be able to join Wi-fi networks and pair bluetooth devices from quick settings as well.
The Android 5.1 update is an incremental update to Android 5.0 which was launched late last year with the new Nexus 6 and Nexus 9. Android 5.0 brought material design to the Android ecosystem and it was light enough to be used on smaller devices with 512MB of RAM. The new update should be available for all devices running stock Android 5.0, in due time, OEMs may follow suite.
Source: Android Blog