“Those sins which are already forgiven directly by the Church’s Power of the Keys” are not sins confessed in the confessional***. The Church’s Power of the Keys refers to Papal and Apostolic authority. Since mortal sins can only be forgiven in the confessional, the sins being referred to here are venial in nature,*** which are forgiven in a variety of different ways, * through the Church’s Power of the Keys*. Such ways include being blessed by a priest, receiving Communion, blessing yourself with Holy Water, making pilgrimages, etc. These are means of forgiveness which have been declared to be so by Papal and Apostolic authority.
Though these sins have already been forgiven, they may not yet have been confessed. “are a sufficient object of confession” means that these sins, though they have already been forgiven, are sufficient to merit going to confession in the first place, even if you don’t have any mortal sins to confess.