Irreversible Process

[Dominique Zuk 1G]

In the book, an irreversible process is explained as an "expansion against an external pressure that differs by a finite amount". Could someone possibly explain this please? I don't really understand the definition.

[Diana_Visco_1l]

An irreversible process occurs due to the finite gradient between the two states of a particular system. In the irreversible process the initial state of the system and surroundings cannot be restored from the final state and increases decisively and it cannot be reduced back to its initial value. Additionally, the various states of the system on the path of change from initial state to final state are not in equilibrium with each other.