Difference between reversible and irreversible processes
In a reversible process, at each point along the process path, the system is only slightly removed from being in thermodynamic equilibrium with its surroundings.  So the path can be considered as a continuous sequence of thermodynamic equilibrium states. For an irreversible process, the system is not close to thermodynamic equilibrium with its surroundings at each point along the path.

At any point along the path of a reversible process, both the system and the surroundings can be returned to their original states without significantly affecting anything else.  This cannot be accomplished if the process is irreversible.