树木 (shùmù) is a collective noun meaning "trees" in a general sense. Unlike 树 (a single tree), 树木 refers to trees as a group or category. For example, 树木很多 means "there are many trees."
公园里的树木长得非常茂盛。
Gōngyuán lǐ de shùmù zhǎng de fēicháng màoshèng.
The trees in the park are growing very lushly.
秋天的时候,树木的叶子都变黄了。
Qiūtiān de shíhòu, shùmù de yèzi dōu biàn huáng le.
In autumn, the leaves of the trees all turn yellow.
城市里应该多种一些树木。
Chéngshì lǐ yīnggāi duō zhòng yīxiē shùmù.
More trees should be planted in the city.