上下 (shàngxià) is a compound word meaning "up and down" or "above and below." It can function as a noun, adverb, or adjective to describe vertical orientation, approximate range, or hierarchical relationships.
电梯上下运行得很快。
Diàntī shàngxià yùnxíng de hěn kuài.
The elevator goes up and down very quickly.
这个孩子大概十岁上下。
Zhège háizi dàgài shí suì shàngxià.
This child is about ten years old.
她紧张地上下打量着面前的陌生人。
Tā jǐnzhāng de shàngxià dǎliang zhe miànqián de mòshēngrén.
She nervously looked the stranger up and down.