A buck-boost transformer is a special kind of transformer that can change the voltage of electricity.
Think of electricity as a bunch of little guys wearing different outfits, with each outfit representing a different level of energy. Sometimes we need more energy (higher voltage) or less energy (lower voltage) for different machines or appliances.
This transformer can help us make those changes. It is like a little machine that the electricity goes through, and it changes the outfits of the little electricity guys to make them have more or less energy.
It is called a buck-boost because it can either "buck" the voltage down, meaning it makes it lower, or it can "boost" the voltage up, meaning it makes it higher. It can also work with both AC and DC electricity.
It's pretty cool because it allows us to use the same electricity source for a bunch of different things without having to worry about the voltage being too high or too low for any of them.