Okay kiddo, a fusilier is a special kind of soldier who carries a long gun called a musket. They were really important in the olden days when people used to fight wars with guns that you had to load one bullet at a time.
The fusilier would stand in a line with other soldiers who also had muskets and they would all aim and shoot at the same time. It was like a big game of "shoot the bad guys".
The fusilier had to be really good at aiming and shooting their musket. They would practice a lot to make sure they would be ready for battle. But being a fusilier wasn't just about shooting. They also had to be tough and brave, because they were on the front lines of battle and had to fight the bad guys up close.
So, in simple terms, a fusilier is a soldier who carried a long gun and was very important in olden day battles. They were part of a group of soldiers who stood in a line and shot at the same time, and they had to be brave and tough in battle.