The characters combine to mean “the heart/mind is straight, and the mouth is fast.” This creates the image of an unimpeded, direct line from thought to speech, explaining why a person with this trait speaks so frankly and quickly.
In Chinese culture, which often prioritizes social harmony (和谐, héxié), maintaining face (面子, miànzi), and indirect communication, being 心直口快 is a complex trait. On one hand, it can be valued as a sign of sincerity (诚恳, chéngkěn) and honesty. A friend who is 心直口快 might be trusted because you know they aren't hiding their true feelings. On the other hand, it can be seen as a social liability. Speaking too directly can easily cause others to lose face, disrupt group harmony, or be perceived as rude and lacking in emotional intelligence (情商, qíngshāng). This contrasts with many Western cultures, particularly American culture, where “telling it like it is” or “brutal honesty” is often lauded as a virtue. While an American might say “I appreciate your honesty,” a Chinese person might think, “They are sincere, but they don't know how to speak properly (不会说话).” Therefore, whether 心直口快 is a positive or negative trait heavily depends on the relationship between the speakers, the setting, and the topic of conversation.
This is a very common conversational term. Its connotation changes with tone and context.
The term is almost exclusively used in informal and semi-formal conversations. You would not use it to describe someone in a highly formal written report.