Explain the difference between reversible and irreversible

The difference between reversible and irreversible is reversible can be changed but irreversible can not.

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.