"I don't believe I understand. What does he mean by 'salvific suffering'? If he is referring to -our- suffering, atoning for sins, didn't Christ fully accomplish that on the Cross?"
Yes, Christ fully atoned for our sins - past, present and future- when He died for us on the Cross. His suffering was salvific (saving).
However, we can, and should, unite our sufferings with His.
These are some excerpts from "Salvifici Doloris"
(The Christian Meaning Of Suffering)by Pope John Paul II (1984)
Encyclical of Pope John Paul II ...declaring the power of salvific suffering, the Apostle Paul says: “In my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the Church.”
These words seem to be found at the end of the long road that winds through the suffering which forms part of the history of man and which is illuminated by the Word of God. These words have as it were the value of a final discovery, which is accompanied by joy. For this reason St. Paul writes: “Now I rejoice in my sufferings for your sake.” The joy comes from the discovery of the meaning of suffering, and this discovery, even if it is most personally shared in by Paul of Tarsus who wrote these words, is at the same time valid for others. The Apostle shares his own discovery and rejoices in it because of all those whom it can help — just as it helped him to understand the salvific meaning of suffering.
Christ did not conceal from His listeners the need for suffering. He said very clearly: “If any man would come after me..let him take up his cross daily,” and before His disciples He placed demands of a moral nature that can only be fulfilled on condition that they should “deny themselves.” The way that leads to the kingdom of heaven is “hard and narrow,” and Christ contrasts it to the “wide and easy” way that “leads to destruction.”
On various occasions Christ also said that His disciples and confessors would meet with much persecution, something which as we know—happened not only in the first centuries of the Church’s life under the Roman Empire, but also came true in various historical periods and in other parts of the world, and still does even in our own time.
This encyclical is not really that long. It brings to light the mystery of suffering.