非 (fēi) can mean "wrongdoing," "fault," or "non-/un-" depending on context. As a noun, it refers to error or misconduct, as in the phrase 是非 (right and wrong). It also serves as a negation prefix in formal Chinese, similar to "non-" in English.
别人的是非我们不要管。
Biérén de shìfēi wǒmen bùyào guǎn.
We shouldn't meddle in other people's gossip.
他这个人最喜欢说别人的是非。
Tā zhège rén zuì xǐhuān shuō biérén de shìfēi.
He is the kind of person who loves to gossip about others.
我们要分清是非,不能糊涂。
Wǒmen yào fēnqīng shìfēi, bù néng hútu.
We must distinguish right from wrong and not be confused.