牌子 means 'sign' or 'brand,' with two common uses: a physical sign or placard that displays information, and a commercial brand or label. Context determines which meaning is intended.
这个牌子的手机质量很好,我用了三年都没坏。
Zhè gè páizi de shǒujī zhìliàng hěn hǎo, wǒ yòng le sān nián dōu méi huài.
This brand of phone is very good quality — I've used mine for three years without any problems.
门口挂着一个牌子,上面写着"请勿入内"。
Ménkǒu guà zhe yī gè páizi, shàngmiàn xiě zhe "qǐng wù rù nèi."
There's a sign hanging at the door that says "Do Not Enter."
你喜欢什么牌子的咖啡?
Nǐ xǐhuān shénme páizi de kāfēi?
What brand of coffee do you like?