The characters combine to form a literal and powerful image: “Mouth says 'yes', but the heart says 'no'.” This structure makes the meaning of duplicity and inner conflict immediately clear.
In Chinese culture, maintaining social harmony (和谐, héxié) and giving “face” (面子, miànzi) can be very important. This sometimes leads to situations where people avoid direct contradiction to prevent embarrassment or conflict. However, 口是心非 goes beyond simple politeness or tact. It is a serious character flaw implying deceit and untrustworthiness. A Westerner might equate this with being “two-faced” or “hypocritical,” and the comparison is quite accurate. The key difference is the cultural backdrop. While a Westerner might prioritize “brutal honesty,” traditional Chinese culture might value a more indirect approach to communication. Despite this, 口是心非 is still universally seen as a negative trait. It isn't used to describe someone telling a “white lie” to be kind; rather, it describes someone with a fundamentally insincere or deceptive character. It points to a moral failing, not a social strategy.
This idiom is common in both spoken and written Chinese and carries a strong negative connotation. You would use it to criticize someone's lack of sincerity.