Signs are a method of placing messages on the game map.
Placing a sign is done through the Landscaping Toolbar. Click on the button and click on a square that you want to place the sign on. A dialog window 'Edit sign text' will appear, allowing you to type something more-or-less useful in. Press OK to store the sign.
To edit an existing sign just click on it to bring up the 'Edit sign text' dialog window again.
To delete an exisiting sign all you have to do is edit it, and clear all text.