You are able to quickly get an audio message out to everyone in the household (or at those within earshot of an Echo speaker) by saying "Alexa, broadcast" or "Alexa, announce" followed by whatever you want to say. So, you could say, "Alexa, announce dinner is ready" and every Echo device in the household will playback "Dinner is ready" using your recorded voice.
The command is also useful for announcing when you're leaving the house, or to announce when you're on the way home. The latter could be accomplished using Alexa on your smartphone or directly from your car if it has full Alexa integration (which is found in some newer vehicles like the recently announced 2019 Toyota Corolla Hatchback).
Announcements expands the already robust communications skills of Echo devices including Calling, Drop In, and Messaging.
Earlier this week, Amazon added the ability for Fire 7 and Fire HD tablets to function in essence as Echo speaker(when plugged into an outlet). In mid-March, the company brought Alexa voice calling and messagingto the Fire, Android and iPad tablets.