Signs are very useful things because they are easier to read than messages using words. They are easier to read because a picture passes through your mind very fast but with words you have to take your time and read every single word on the sign. Signs like "no parking" signs are not the only signs we use. In fact there is a whole language where people use expressions and signs with their hands. This language is called ASL. It was invented for people who are deaf, because they can see but not hear.
The main character in the story we read was deaf and he used ASL to communicate with his family. In the book, the main character passes on a message about where slaves can hide next. He does this by painting a picture of the next safehouse on the inside of an egg shell and giving it to a slave who has come to the market to do some shopping.
Being deaf was an advantage in this situation because the main character was forced to deliver the message without talking, which made it less likely that the slave catchers would understand him passing on the message.