腿 (tuǐ) is a noun meaning "leg." It refers to the lower limbs of a person or animal and appears in many common expressions. For example, 跑断腿 is an exaggerated phrase meaning "to run one's legs off."
跑了一天,我的腿都快断了。
Pǎo le yī tiān, wǒ de tuǐ dōu kuài duàn le.
After running around all day, my legs are about to break.
她的腿受伤了,走路有点不方便。
Tā de tuǐ shòushāng le, zǒulù yǒudiǎn bù fāngbiàn.
Her leg is injured, so it's a bit inconvenient to walk.
小猫的腿很短,跑起来特别可爱。
Xiǎo māo de tuǐ hěn duǎn, pǎo qǐlái tèbié kě'ài.
The kitten's legs are very short, and it looks especially cute when it runs.