This is the 1473rd most frequent Dutch word.
stille
"Stille" means "silent" or "quiet" in English.
Here, 'stille' is used as an adjective to describe the night as quiet or silent.
De stille nacht bracht rust voor iedereen.
The quiet night brought peace for everyone.
In this instance, 'stille' is an attributive adjective describing the observer as quiet or reserved.
Hij is een stille observator in de klas.
He is a quiet observer in the class.
In this sentence, 'stil' (similar to 'stille') is used as an adjective to describe the park as silent.