The noun 叶子 (yèzi) means 'leaf,' the flat green part of a plant. It is the colloquial form with the noun suffix 子, while the more literary form is simply 叶. It is used in everyday language when talking about plants, trees, and nature.
秋天到了,叶子都变黄了。
Qiūtiān dào le, yèzi dōu biàn huáng le.
Autumn has arrived, and the leaves have all turned yellow.
地上落了一层厚厚的叶子。
Dì shàng luò le yī céng hòuhòu de yèzi.
A thick layer of fallen leaves covers the ground.
她捡了一片漂亮的叶子夹在书里。
Tā jiǎn le yī piàn piàoliang de yèzi jiā zài shū lǐ.
She picked up a beautiful leaf and pressed it in a book.