What does the "S" on Superman’s chest represent?

The "S" on Superman's chest is actually the family crest of the El family on Krypton, and it's all Marlon Brando's fault. In 1978's "Superman," Brando wanted his character Jor-El (Superman's dad) to have the same "S" on his costume as Superman, but up until that point the symbol really stood for nothing beyond just being a symbol of the hero. But when one of Hollywood's most bankable stars makes a request, you try to make it work. So filmmakers decided it was now a family crest, and while its origin was a little wonky, it did eventually get merged into DC canon and they even built on it. Now, the "S" isn't just the family crest, because the family crest represents "hope." Comic continuity is basically a game of telephone played across decades.