When Moishe the Beadle returns to the town of Sighet, he has a terrible tale to tell. After being deported from the town as a foreign Jew, Moishe was handed over to the Gestapo at the Polish border along with the other Jews. There, in a scene of unimaginable horror, they were systematically murdered by the Nazis and buried in the mass graves which they had been forced to dig.
Miraculously, Moishe somehow managed to escape. Now that he's made his way back to Sighet, he's anxious to warn the Jews living there of what's in store for them. However, no one wants to listen to him; everyone's convinced that Moishe's crazy, so they don't believe a word he's saying.
Here, we have a prime example of dramatic irony. This is a literary technique whereby we, the reader or the audience, know something that certain characters don't. In this case, we know, with the benefit of hindsight, that the horrific scenes described by Moishe took place all over occupied Europe during the Second World War.
We know, then, that what he's saying is absolutely true. Sadly, the Jews of Sighet don't know this, and when they find out that Moishe was right all along, it'll be too late.