In the sentence, rain is compared to the key, which expresses the importance and function of rain for the arrival of spring. The significance of metaphor is to emphasize the key role of rain on plant growth, earth recovery and seasonal changes. As a tool to open the door, the key symbolizes that the role of rain is to open the door of spring and make nature full of vitality. It is often used in poetry and literary works to make the description more emotional and vivid through image metaphor.