Okay kiddo, so back in a long time ago in Europe, there was this thing called "feudalism". It was when the king owned all the land in a kingdom and he would let his trusted knights and lords manage pieces of that land.
So, let's say the king owned a big piece of land called a "manor". He would give parts of it to his knights and lords as a reward for their loyalty and military service. These pieces of land were called "fiefs".
The knights and lords who got these fiefs were called "vassals". They had to promise to be loyal to the king and do some things for him, like fight in battles if needed. They also had to promise to be fair to the people who lived on their fiefs, like farmers and peasants.
The people who lived on the fiefs were called "serfs". They couldn't leave the fief without permission and they had to work for the vassal who owned the land. They farmed the land, raised animals, and did other kinds of work.
The vassals, in return, would protect the serfs from harm and make sure they had everything they needed to survive.
So basically, feudal land tenure is when the king owns all the land and gives it to his knights and lords who promise to be loyal to him and protect the people who live on their fiefs. The people who live on the fiefs are called serfs and they have to work for the vassals, but the vassals have to protect them and make sure they have what they need to survive.