A secret message is designed for an absolutely private and secure purpose. It requires you entering BigAnt password to read it.
Send and read messages
Double click the target contact to open the chat window, click the icon 'send a secret message' in input box, the icon with highlighted blue color shows that your typing message will be sent out as a secret message.
Read a secret message
A secret message won't be readable in recipient chat window, instead it requires entering your password to read it.