Google launched a feature called "Smart Lock" in Android 5.0 that allows the smartphone to be paired with nearby devices (such as laptops, smartwatches, and so on) through NFC or Bluetooth, and then stay unlocked for as long as the connection is maintained.
This way, when your smartphone or laptop is on you and you have a smartwatch, you can use your device instantly without having to enter its PIN or pattern anymore. This feature can't be used for all situations (you might want to keep the phone locked at the airport, for example), so Google is now offering yet another smartphone locking feature called "Trusted Voice."
As you can already imagine from the name, this feature is about allowing smartphone owners to unlock their devices with a voice command. Presumably, the feature is a little more advanced than simply listening to the words being said; it will actually analyze voice patterns.
Either way, ironically, it seems that even Google doesn't trust Trusted Voice to be very secure. It warned users that someone else could be unlocking their devices if they have a recording of the user's voice, or even a similar voice. This is why voice is likely the least secure method of biometric authentication among fingerprint, face, iris and heartbeat recognition. It's just too easy these days for someone else to record your voice.
If Google has made it advanced enough so at least the people around you can't unlock your phone just by hearing you say the command, then it could be more secure than simply "sliding to unlock," but almost as convenient.
Face Unlock, another authentication feature in Android that was added a few versions back and has improved significantly since then, could be a good alternative to Trusted Voice. It is perhaps slightly more secure, and even more convenient, because you only have to look at your phone for a second or two to make it work.
However, in some cases Trusted Voice could be even more secure than face or fingerprint recognition. For instance, someone could physically force you to unlock your device with your face, iris or fingerprint, but they can't do that as easily if you need a PIN, passphrase, pattern or even a voice command.
The rollout of Trusted Voice has just begun as an update to the Play Services framework, so it might not arrive to all users at the same time.