It's an adverb that modifies adjectives and adverbs, indicating a moderate degree or intensity.
Here are some examples:
* "Es ist ziemlich kalt." (It's quite cold.)
* "Der Film war ziemlich langweilig." (The movie was pretty boring.)
* "Ich bin ziemlich müde." (I'm pretty tired.)
"Ziemlich" can be used in both formal and informal settings. It's a very common word in German and is often used to express a neutral opinion or feeling.