Reversible and Irreversible processes

[Pratyusha_M_1M]

Can someone please explain reversible and irreversible processes?
I don't understand the 'infinitesimal change' part.

[Jenny_Thompson_3I]

A reaction that is reversible occurs when the external and internal pressures are the same and the system is at equilibrium (for example 1.00 atm), therefore when the system expands it does so when there are extremely small changes in the external pressure that is reestablished when the system expands. An irreversible system occurs when there is a difference between the external and internal pressures, or the system is not at equilibrium (external pressure is 1.00 atm and the internal pressure is 2.00 atm) when work is done, the expansion is very rapid and cannot be reversed due to the initial differences in pressure. I hope this helps!