Acceleration due to gravity is the rate at which an object accelerates towards the Earth when it is in free fall. On the surface of the Earth, this acceleration is approximately 9.81 m/s². This means that for every second an object falls, its speed increases by about 9.81 meters per second. This acceleration is caused by the gravitational force exerted by the Earth. All objects, regardless of their mass, experience the same acceleration due to gravity when air resistance is negligible. This principle is a key concept in physics and is essential for understanding motion and forces in our universe.