In reading Ariel Dorfman's poem "Hope," from his collection of poetry In Case of Fire in a Foreign Land, the elements that seem most important to me are first that a son is missing. We learn that this state of affairs has been happening since the previous year when the son was taken on May 8. Next we are told that he was "taken"—he did not leave on his own. Against his will, the son was placed in an unmarked car, without license plates, with promises by those who took him that he would only be gone for a "few hours" for "routine questioning."
As the poem continues, we find that the parents have been unable to find out anything about him: whether he was dead or alive, since May 8. The speaker is horrified by these events as well. He asks in what country, in what world, could such a thing take place.
However, on this day, the day the speaker is presently observing, things have changed dramatically. A friend or neighbor (a compañero) has come to visit. He also had been held prisoner in the "red house" at Villa Grimaldi, the house once owned by the Grimaldi family. (We can assume they no longer live there—perhaps it has been seized by the "police.") While there (and this man has only just been released that day), the compañero heard their son's voice, which he recognized—and their son's screams.
Ironically (and at first, surprisingly), the parents do not lament the torture of their son, but are filled with hope (hence the poem's title). Eventually we understand that this is because they know that as of that day, he has been alive the entire time he has been missing, and is alive still. We also know that their hope extends to "next year," when after eight months they hope they may again hear news of his torture and know—then as well—that he is still alive. We grasp the fact that for them, death is the worst piece of news they could receive. Their son's suffering helps them through their own suffering by giving them something to pray for, to hope for—his continued life.