Though you know this book as Slaughterhouse-Five, the full title is actually Slaughterhouse-Five, or the Children's Crusade: A Duty-Dance With Death. When main character Billy Pilgrim winds up in Dresden, Germany, as a prisoner of war (POW) in World War II, he and a hundred other American POWs are kept in an abandoned slaughterhouse called Slaughterhouse-Five. That is the strict plot-level meaning of the title. But we can't ignore the larger metaphor of the title: after all, this is an anti-war book, and what is war except slaughter?
There is also a third level of meaning to the title, which is biographical. Obviously, this book is fiction: there's plenty of aliens and time travel to go around. But there's also a ton of biographical detail from Kurt Vonnegut's own life in these pages, including the fact that he was (yes, you guessed it) an American POW in the city of Dresden during the infamous Dresden firebombing.
Vonnegut survived the bombing by sheltering in an underground meat locker on the grounds of the slaughterhouse where he was a prisoner (source: Klinkowitz, Jerome. Slaughterhouse-Five: Reforming the Novel and the World. Boston: Twayne Publishers, 1990. Pg. 3). So Slaughterhouse-Five is: (a) where Billy Pilgrim, the main character, winds up during the war; (b) figuratively, what war is; and (c) where Kurt Vonnegut, author, actually spent several months at the end of World War II.
Then there is a second part to the title, The Children's Crusade. The Children's Crusade was a real historical event and also a giant wartime screw-up. Fired up by the religious fanaticism of the day (by which we mean thirteenth-century medieval Europe), a boy named Nicholas Cologne inspired thousands of children and teens to marched out of France and Germany to go Jerusalem to join the Crusades. It's unclear if any of these kids ever made it to Jerusalem; many turned back and it's likely that most of them died along the journey (source). So the Children's Crusade, a pointless sacrifice of innocent life, relates to the novel's anti-war themes.
The Children's Crusade has heavy symbolic weight in this particular book. The narrator (a Vonnegut stand-in) says that he promised the wife of his war buddy that he would call his war book The Children's Crusade so that it would never be misinterpreted as a heroic war story (1.11). Slaughterhouse-Five may be about war, but it sure as hell ain't about heroes. It's a book about innocents sent to fight a war they do not understand, who suffer terrible things for no reason. This sounds a lot like a Children's Crusade to us.
Last but not least, how about that Duty-Dance With Death? The narrator is quoting the French writer Céline here, who said that all art depends on a dance with death. Céline, who fought for France in World War I, claims that he has spent his entire life "waltz[ing death] around" (1.20.2). Billy Pilgrim also spends most of his life engaging with death – seeing it in his dreams, traveling back to it in time, trying to avoid it with the Tralfamadorians. But neither Billy's nor the narrator's dance with death is voluntary. Both of them have fought in a war beyond their control and not of their choosing. So this is not a willing dance with death; it's a duty-dance with death.