The sounds in the dark forest were very frightening.

frightening 是动词 frighten 的现在分词形式。frighten 由名词 fright（恐惧）加上动词后缀 -en 构成，意思是"使害怕、使恐惧"。frightening 作为现在分词，在这里表示"令人害怕的、吓人的"。