When something is sacred to you or to a group of people, that means it's part of your religion, or you think it's closely connected with a god, or you just think it's really, really special and important, so you treat it in a very serious way.
For example, for many Christians, Easter is a sacred holiday. On Easter, they celebrate the time when, according to their sacred book, the Bible, Christ arose from the dead. The book, the story of the resurrection, and the Easter holiday are all sacred to many people, which means they treat all those things with honor and respect.
So far, we've seen that stories, books, and holidays can be sacred. Places, too, can be sacred. If I think of a cemetery as sacred, that means I believe it's an important place where some people I love have been laid to rest. If I treat the cemetery as sacred, that means I speak softly and walk softly there. I don't laugh loudly there or crack jokes, and I don't bring in any snacks or pets. It's a sacred place, so I treat it respectfully.
What else can be sacred? Words, names, music, food, clothing, and more. Your promises can be sacred, meaning they're very serious, and very important. If you become a doctor, you take a sacred oath (or promise) to heal people and never harm them. Here's Homer taking a sacred oath to protect the secrets of the Stonecutters.
In stories, you might read about a hero accepting a sacred task or duty: a very serious, very important job. Or the hero might go on a sacred quest or a sacred journey: one that's deeply important, too important to forget about or even joke about. In The Lord of the Rings, Frodo fulfills a sacred duty to destroy an evil ring and save his entire world.