静悄悄 is a reduplicated adjective (状态词) that vividly describes a scene of complete silence or stillness. You can use it as a predicate with 的 (e.g., 教室里静悄悄的 'the classroom is very quiet') or as an adverb with 地 to describe doing something silently (e.g., 静悄悄地走进来 'come in quietly'). It emphasizes an atmosphere of absolute quiet, often implying that no one is speaking or making noise.