It just means the author’s intent is ignored in interpreting the work. Like if you, the reader, decide “the sky is gray” is a reflection of the main character’s inner turmoil, that’s what it means. Even if the author was just foreshadowing some rain, your interpretation is correct because death of the author means the reader’s interpretation is the correct one. It’s kind of silly, but it also lets people find new meaning in art and I think that’s neat.
People use it as “enjoy the art but fuck the artist” but I don’t think that’s entirely accurate, unless they’re choosing to interpret certain parts of the books as not coming from a problematic place.