A shield is a board made of wood or a metal sheet. In the past, it protected a warrior from attacks. Today, signs usually bear writing or a pictogram. Both are meant to explain something to us. The word also has other meanings.
Shields as protective weapons already existed in ancient Egypt, for example. They were woven from willow and covered with cloth, later with leather. Later still, a board was used as a shield, then it was reinforced with metal until it was finally made entirely of metal. A handle was attached to the back. In this usage one says "the shield" and in the plural "the shields".
In the Middle Ages, shields were also used to display the warrior's coat of arms. This was important to identify who was fighting on which side. Family coats of arms sometimes show a knight on a horse with a shield. This is a full coat of arms. The shield is only a small part of it, just what the knight is holding in his hand.
Today, shields are mainly known with writings or symbols that refer to something or want to forbid something. In this sense we say "the shield" and in the plural "the shields". We know them from the street, for example. Prohibition signs there are round, instruction signs are square.
On houses there are many signs indicating a shop or a restaurant. This is the origin of today's neon signs. Everyone is certainly familiar with the "Do not enter" signs that you see here and there.