In a game, sometimes you need to move objects around. But sometimes you don't just want to move the object to a new location, you want to turn it around too. For example, think of a puzzle game where you have to fit different shapes into a certain space. You need to be able to move the shape around and flip it over to make it fit.
A rotation system is a way of organizing all the different ways you can flip and turn an object. Imagine you have a block in front of you. You can turn it so the flat side is facing up, or you can turn it so one of the sides is facing up. You can also flip it over so the bottom is on top. There are lots of different ways you can change the orientation of the block.
A rotation system is a fancy way of keeping track of all those different orientations. Sometimes people use letters or numbers to describe each orientation, like "A" is when the block is upright with the flat side facing up, and "D" is when the block has been flipped over. Other times they might use symbols, pictures, or even little animations to show you all the different orientations.
Using a rotation system can be really helpful in games where you need to quickly move and flip objects. Instead of having to try every possible orientation until you find the one that fits, you can use the rotation system to quickly get the object into the right pose. It can save you a lot of time and frustration!