The release was detailed on the Official Android Blog, and should start rolling out to Nexus devices this week. Don't get too excited; this release is mostly about fixing the plethora of bugs in Android Lollipop. There are four new features worth mentioning, though.
Multi-SIM support: Have a phone with more than one SIM slot? Now you can use them both!
HD Voice: On networks with HD Voice like Verizon and T-Mobile, you can now actually use it on supported devices (such as the Nexus 6).
Device Protection: If your phone is lost or stolen, you'll need to log in with your Google account to unlock it—even if it is factory reset.
Quick settings: Select which Wi-Fi network to join or Bluetooth device to pair with, right from a drop-down list in the quick settings menu.
Most of those features won't be of much use to people who don't use stock Android phones (Nexus, Google Play Edition, or Android One). Most manufacturers enhance Android with their own quick settings menus, support for carrier features like HD Voice, or dual-SIM support. But the new device protection feature should be a welcome deterrent to theft for all phones that ship with Android 5.1.
Perhaps the best to come from its release will be more rapid updates from manufacturers, who have not been as speedy in updating their phones to Android 5.0 as we hoped. This is in part due to all the bugs in the initial release—this major bugfix release might help grease the wheels on Android updates.