Classical mechanics is a science that deals with how things move, like balls, cars and airplanes. It's been studied for a very long time (like since the ancient Greeks), and some really smart people like Galileo, Newton, and Euler made big discoveries about it.
Galileo noticed that objects fall to the ground at the same rate, no matter how heavy they are. He also figured out that things keep moving at the same speed and direction until a force makes them change.
Newton took Galileo's ideas a step further and discovered three laws that explain how things move: 1) things that are not moving won't start moving unless some force makes them move; 2) things that are moving will keep moving in a straight line at the same speed unless some force makes them stop or change direction; and 3) for every force, there is an equal and opposite reaction force.
Euler came up with equations (fancy math stuff) that showed how objects move and how they respond to forces. These equations make it possible to predict how things will move and to design machines like cars, airplanes, and rockets.
So now people use classical mechanics every day to build things that move, to understand how to make things go faster, and to explore space. It's pretty cool!