天上 (tiānshàng) is a noun phrase meaning 'in the sky' or 'up in the heavens.' It is used to talk about things located in or coming from the sky, such as stars, clouds, or airplanes.
天上飘着几朵白云,真漂亮。
tiān shàng piāo zhe jǐ duǒ bái yún, zhēn piàoliang.
A few white clouds are floating in the sky; so pretty.
你看,天上有一架飞机飞过去了。
nǐ kàn, tiān shàng yǒu yī jià fēijī fēi guòqù le.
Look, a plane just flew across the sky.
小时候奶奶说天上住着神仙。
xiǎoshíhòu nǎinai shuō tiān shàng zhù zhe shénxiān.
When I was little, Grandma said immortals lived in the sky.