Meaning:

“人” means “human,” “畜” means “beast,” and “无害” means “harmless.” This term, which literally means “harmless to humans and beasts,” originally appeared on the package of certain products making clear that the products are completely safe to use. Nowadays, however, it’s used by Chinese netizens to refer to those who appear innocent and kind-hearted.

Example:

A: 为什么走在街上总有人往我手里塞传单？

Wèishénme zǒu zài jiēshàng zǒng yǒurén wǎng wǒ shǒulǐ sāi chuándān？

Why do strangers always give me pamphlets on the street?

B: 可能因为你长了一张人畜无害的脸吧。

Kěnéng yīnwèi nǐ zhǎng le yīzhāng rénchù wúhài de liǎn ba。

Perhaps because you appear to be innocent and kind-hearted.