Metaphors are the part of language where a word or phrase is used to suggest a likeness or similarity between it and another situation. It is not literally correct, but is figurative. For example: An angry summer sun turned Hong Hong into an oven. Even if the temperature is over 40℃, an oven would be between 100℃ and 250℃. Also the sun is never angry nor happy. The comparison is exaggerated to show how hot the weather was in Hong Kong this past summer.
Dead metaphors are those whose comparison the speaker does not understand. It might be understood by someone, just not the speaker. A Pyrrhic victory may be a conditional dead metaphor. I understand it, but to someone who does not it is meaningless. A Pyrrhic victory means succeeding at something, but at too great a cost. For example, working very hard for a promotion, but then having a heart attack because of stress is a Pyrrhic victory. (Google King Pyrrhus of Epirus and the battle of Asculum in 280 BC or read Plutarch's The Life of Pyrrhus.) It is an error to use a metaphor that one does not understand.
The whole nine yards means everything. For example, When he cleaned the flat he did the whole nine yards. (He cleaned everything in the flat including the windows and the ceiling.) Nobody knows where this phrase came from, although people have guesses. It is a dead metaphor.
Some conditional dead metaphors get reused in ways that make no sense. For example, a tough road to hold, makes no sense whatsoever. It used to be, a tough row to hoe. It was a farming metaphor for a difficult task. However people who did not understand it, repeated it erroneously. Now it has become an error and an error that makes learning English confusing.
English uses metaphors and they are difficult to learn as a second language, if the context is not understood. Sometimes the original metaphor has been forgotten or even garbled by native speakers. When that happens it becomes a dead metaphor. It is best not to use any metaphors when speaking to people who don't understand them. It is definitely poor style to use a metaphor that one does not understand oneself.