If you have a PIN or password set on the device, you must unlock it to enable all Alexa hands-free features. You can also opt to disable all of these features (or control whether it works while the device is password locked) in the Alexa settings menu. And if you want your tablet to respond less often when there's another Alexa-enabled device nearby, you can change that with the "Tablet ESP Behavior" option in Alexa settings.
This is a nice upgrade for all Fire 7 and Fire 8 HD tablets running software version 184.108.40.206 or later. It turns a regular tablet into a hands-free smart home control center. This handy feature was already available on Amazon's Fire 10 HD tablet, so it's nice that the company is rolling it out to older models.