A mechanical system is like a big puzzle made out of parts that fit together to make something move. It's kind of like when you build with Legos, but on a bigger scale.
For example, think about a bicycle. A bicycle has lots of different parts that work together to make it move. You have the wheels that turn, the pedals that you push with your feet, and the chain that connects the pedals to the wheels. But that's not all! You also have brakes that help you stop, gears that let you go faster or slower, and a frame that holds everything together.
All of these parts work together to create a mechanical system that makes the bike move. And just like with a puzzle, if one of the pieces is missing or doesn't work right, then the whole system won't work correctly.
Another example of a mechanical system is a car. A car has an engine that runs on gasoline or electric power, and that engine is connected to the wheels with a transmission. The wheels are attached to the car with a suspension system and steered with a steering wheel. You also have brakes, lights, and other systems that help the car operate safely.
So basically, a mechanical system is a bunch of parts that work together to create motion, whether that be a bicycle, a car, or something else entirely.