https://www.cnbc.com/2018/10/27/how-to-turn-on-amazon-alexas-whisper-mode.html

Amazon recently rolled out an update to its Alexa voice assistant that lets you turn on a new "whisper mode." When it's active, you can whisper to an Amazon Echo so that Alexa quietly answers you.

It's convenient if you use Alexa in a bedroom and don't want to wake up your partner at night, or even if you have a question to ask while in the living room but would prefer if the whole house didn't hear the response.