Do you know what a shovel is? A trencher is like a special kind of plate that looks like a shovel. It is not used to dig, but to hold food when you eat.
A long, long time ago, people didn't have fancy plates like we do now. They would use big pieces of bread or dough to hold their food. This was called a trencher.
Later, when people started using plates, they would sometimes still use the word "trencher" to describe the plate that held their food.
Nowadays, a trencher is a special kind of tableware that has a deep, hollow part to hold food, and sometimes a handle or a stem so that you can hold it easily. It's often used for things like soup or stew because the hollow part helps keep the food from spilling.