The carry forward rule is a provision that allows Members to contribute unused amounts as concessional contributions.
As of the 1st of July 2018 if your total superannuation balance is less than $500,000 you are able to add unused amounts to your original concessional contributions cap of $25,000 ($27,500 from 1 July 2021).
John is an employee and only receives super guarantee contributions from his employer. Over the four years, John has a significant unused portion. John could utilise carry-forward concessional contributions to use any of this unused cap by making addition contributions into his super. In financial year 2021/2022, John could make a $80,500 concessional contribution to super on top of the $6,000 he would receive from his employer.
The ATO have a full index of listings regarding the concessional contributions cap amount and unused amount. For more information please refer to the ATO website here.