A Whisper is an option you can use to chat with other toons. Using whisper, other toons cannot see it unless it is the toon you are whispering to.
How to whisper
To find whisper, click on the toons name, then you will see a toon panel of them. The third button is where you will see the option to whisper. If you click on it you will see a popup to the top left of your screen. Click the blue button or start typing and you should be ready to whisper once you click enter and send it to them.