That sign is a rectangle. It has 4 sides and 4 corners. It is shaped like a triangle. Children can also learn to create shapes. Ask your children to draw the shape of the street sign in the air.
Children learn that people communicate with symbols.
Again, traffic signs are a good place to start. You might point out how the shape of the street sign helps people know what the sign means. The link between symbols and the concepts they represent is not always clear to children. Keep restating the meaning, but don't push. Children will make the connection when they are ready.