The thinking is that there are Saints that are/were so meritorious that they actually built up merits above and beyond what is required for satisfaction. These are collected in what is called "the treasury of the Saints" or "the treasury of merits." These merits, by means of indulgences, can be purchased and applied to the account of others to pay for their temporal sin(s).

This, of course, suggests that the merits of Christ are not enough. . .Luther was, in part, responding to this practice when he wrote his 95 Theses.

Trust the past to God's mercy, the present to God's love and the future to God's providence." - St. Augustine
Hiraeth