Signs are nametags attached to buildings and vehicles. They are immaterial like notes (so you don't need resources to make one) and have a limited length (of how many characters?). If the sign is different from the dynamic name you have given to the target, you can see the text of the sign on the Buildings & Vehicles page under the dynamic name, written in light blue font.
How to change or add a sign
Clicking on the dynamic name of a building or a vehicle leads to a page where you can rename it and it also shows all the signs attached to it. It will say if you are carrying any tools that can be used for altering signs. If you are, it will say "SETUP SIGN PROJECT" and
"To set up a sign alteration project select the type of project from the list on the left and fill in the required details on that row. Signs in position 1 will be used as the default name for the building or vehicle. Signs in other positions are visible on the buildings page."
Then you can select to change the text on one of the existing signs or to create a new one.
Starting a sign project will create an event that's visible to all characters in the location.