The value 9.8 m/s^2 is the average acceleration of a falling object due to the force of gravity on Earth. The letter g represents this value the formula v=gt. With this constant and formula, the speed of an object is calculated at the time t in seconds after it is dropped.
All objects fall at the same rate of speed due to gravity, regardless of their weight. If a small grain of sand and a bowling ball are dropped at the same time, both hit the floor at the same time. The value of g varies by one's location on the planet, ranging from 9.782 m/s^2 in the Panama Canal Zone to 9.832 m/s^2 at the North Pole.