Ok kiddo, so imagine you have a big group of friends and you are the leader of the group. You make the rules and everyone follows them.
Now, imagine that you have some friends who are also leaders of smaller groups. These leaders are called vassals. They look up to you and follow your rules, but they also have their own rules and people to lead.
In the Kingdom of Jerusalem, there was a big leader who was called the king. He had his own rules that everyone had to follow. But there were also other leaders in different cities and areas who were vassals of the king. They still had to follow the king's rules, but they also had their own people to rule and their own laws to follow.
The vassals of the Kingdom of Jerusalem were kind of like mini-kings in their own territories. They had to respect the king and help him when he needed it, but they also had their own responsibilities. For example, they had to protect their own people from enemies, make sure they had food to eat, and collect taxes.
So, being a vassal was kind of like being a smaller leader in a bigger group, but you still had to listen to the big leader and do what was best for everyone.