Google has several options for intelligently locking your home screen already, including trusted places or devices. Now, it's adding a new option: trusted voice. If you say "OK, Google", your phone can unlock, but someone else's voice won't unlock it.
Smart Lock is a convenience feature more than it is a security feature. As such, Google will give you a warning if you try to activate it that this method is less secure than a PIN, password or pattern lock. If someone has a recording of your voice saying "OK Google", they could theoretically unlock your device.
However, on the off-chance you don't have clever international spies following you around recording your voice to gain physical access to your phone, this method can be handy to have. You can unlock your device with a voice command, and jump straight to giving it other commands if you'd like. As Android Police points out, the feature is still rolling out to some users via Play Services, so it's unclear just how many people will have access to the option in what time frame. If it's showing up for you, try it out and let our other commenters know how it's working!