na-adjective
quiet; still; secret; surreptitious
Describes a quiet, calm atmosphere or an action done secretly, without drawing attention. Often used for hushed places or discreet behavior.
夜の森は密やかで、風の音だけが聞こえた。
The forest at night was quiet, and only the sound of the wind could be heard.
彼
He quietly slipped out of the room.
二人は密やかな恋を続けていた。
The two continued their secret love affair.
静か is the common word for 'quiet' in general. 密やか adds a nuance of secrecy or being hidden, not just absence of noise.
こっそり is an adverb for doing something secretly or stealthily. 密やか can describe both the quiet atmosphere and the secretive manner.
The word is composed of 密 (secret, dense) and the adjectival suffix やか. The exact historical derivation is uncertain, but the kanji reflects the core meaning of secrecy and quietness.