Okay kiddo, imagine you live in a village back in olden times. The village is ruled by a king and he needs people to help him run and protect the village. He chooses some of the villagers who are strong and brave to help him out. These chosen villagers are known as thegns.
Basically, the thegns were like the king's right hand people. They helped him make important decisions, protected the village from enemies and often owned land or had important jobs in the village. They were kind of like the king's managers and bodyguards.
Being a thegn was a pretty big deal back then, and only certain people were chosen to become one. It was a sign of trust from the king, and they were rewarded with land, wealth and other privileges.
So in short, the thegn was a trusted advisor and protector of the village or kingdom. They were kind of like the king's personal assistants or bodyguards.