静静的 is the reduplicated form of 静 (jìng, quiet) with the suffix 的, used as an adverbial phrase meaning 'quietly', 'in a quiet manner', or 'still and silent (describing a scene)'. The reduplication softens the tone and adds a gentle, continuous sense. It often appears at the beginning of a sentence to set the atmosphere, or directly before a verb. This form is characteristic of Chinese stative adverbs and is common in descriptive writing.