An object is in free fall when the only force acting on it is gravity. Near the surface of the Earth its acceleration is g = 10 m/s2. This means its speed increases by 10 m/s every second. A stone dropped from rest is moving at 10 m/s after 1 s, 20 m/s after 2 s and 30 m/s after 3 s. In real life air resistance reduces the acceleration, but the value 10 m/s2 is used when air resistance is ignored. Then a heavy ball and a light ball fall together.
You should also be able to estimate the size of everyday accelerations. A person starting to walk accelerates at about 0.5 m/s2, a cyclist speeding up at about 1 m/s2, and a car pulling away at about 2 m/s2. A car braking hard slows down at about 8 m/s2, which is close to the acceleration in free fall. Estimates like these only need to be the right order of size.