Whispers are internal notes that your team can use to chat within individual Conversations. Whispers appear in the Conversation thread in a dotted line bubble, as opposed to the contact-facing messages in a solid line bubble.
To Whisper, click the Whisper icon in your Message Box.
Then, you can type your message. Alternatively, you can just enter /whisper in the message box before typing your note.
The message box will turn yellow, and the Send button will become a Whisper button. Any agent will be able to see the Whisper and respond to it.
Watch the tutorial video on Using Whispers video.