As with most -- all? -- papal encyclicals, the title is taken from the first words of the document -- in this case, spe salvi facti sumus, which is a quote from Romans 8:24.
“SPE SALVI facti sumus”—in hope we were saved, says Saint Paul to the Romans, and likewise to us (Rom 8:24). According to the Christian faith, “redemption”—salvation—is not simply a given. Redemption is offered to us in the sense that we have been given hope, trustworthy hope, by virtue of which we can face our present: the present, even if it is arduous, can be lived and accepted if it leads towards a goal, if we can be sure of this goal, and if this goal is great enough to justify the effort of the journey. Now the question immediately arises: what sort of hope could ever justify the statement that, on the basis of that hope and simply because it exists, we are redeemed? And what sort of certainty is involved here?
The full encyclical is here. Like all of Joseph Ratzinger's writing, it will take some time to digest, but will be well worth reading. His first encyclical, Deus Caritas Est, is here.