Perhaps the main idea in Wiesel's speech is the idea that we must remember the atrocities of World War Two, and specifically of the Holocaust, to avoid ever committing the same atrocities again. One metaphor Wiesel uses to impress this idea upon his audience is:
The memory of evil will serve as a shield against evil ... the memory of death will serve as a shield against death.
In this metaphor, Wiesel presents memory as a "shield" which, he says, people must use to protect themselves from "evil" and "death." His meaning here is that we must remember the atrocities of the Holocaust so that we can prevent them from happening again, and thereby protect ourselves from the "evil" and mass "death" that characterized the holocaust.
Later in his speech, Wiesel uses another metaphor to convey a similar idea about the importance of remembering. He says:
For the first time in history, we could not bury our dead. We bear their graves within ourselves.
The "dead" that Wiesel refers to here are the victims of World War Two and the holocaust. When he says that "we could not bury our dead," he means this literally and metaphorically. Those who died in the gas chambers of Nazi concentration camps, for example, could not be buried respectfully by their loved ones, and neither could, or should their memories be metaphorically buried, or forgotten, afterwards. When Wiesel then says that "we bear their graves within ourselves," he means that we will always carry the memories of their deaths with us. The "graves" here metaphorically represent the deaths of the victims of the Holocaust, and at the same time they also metaphorically represent the memories of those victims. Wiesel is here emphasizing our obligation not to forget these victims.
Wiesel also uses metaphors in his speech to convey the horrors of the Holocaust. For example, he says that during this period, mankind
succeeded in building an inverted Tower of Babel, reaching not toward heaven but toward an anti-heaven.
This "inverted Tower of Babel" is a metaphor representing the idea that mankind no longer seemed to reach up to heaven but instead started reaching down to hell. This metaphor vividly portrays the moral descent of civilization which Wiesel is urging his audience to redress.