Final answer:
In Spanish, the word "buen" means "good." In the sentence, it describes the weather, expressing that it is pleasant.
Step-by-step explanation:
In the sentence ¡Qué azul está el cielo! Hace buen tiempo hoy, the underlined word buen means good. Therefore, the sentence translates to "How blue the sky is! The weather is good today." In this context, "Hace buen tiempo hoy" means that the weather is pleasant or nice.
Learn more about Weather expressions in Spanish