If you want to hide the whole message you can use the "!hidden" flag, add !h or !hidden anywhere in your message and only the GM and the owner of the message will be able to see it. If you use it as a player you and the GM will see the message. If the GM uses it when sending a message as a character then the GM and the player's owner will see it.

You can also send a message where just the roll is private but the result is visible, do this using the !private flag, add !p or !private to your message to make it private.

You can also roll dice privately by rolling as a private character. To do this set the character to private in their settings then select the character in the "Post As" select box in chat. Rolls made as this character will only be visible to the character's owner and the GM.

See Dice Rolls and Messages for more info on dice and on other flags.