Yetholm-type shields are very strong shields that people used to use a long time ago to protect themselves from weapons like swords and arrows. They are also sometimes called "beehive" shields because they look like a beehive with lots of layers of wood stuck together.
To make a yetholm-type shield, people would start by cutting thin pieces of wood, like a pencil, and then they would stick them together in circles with glue or nails. They would make lots of these circles and then stick them all together to make a really thick shield.
The reason this shield is very strong is because when someone hits it with a sword, the layers of wood make it hard for the sword to pierce through to the person behind the shield. The shield is also curved, which makes it easier to hold and move around.
So, think of a yetholm-type shield like a really strong and tough shell that keeps someone safe from getting hit by swords and arrows.