When combined, `心情 (xīnqíng)` means “heart-state” or “mood.” Adding `不好 (bù hǎo)`—“not good”—creates the direct and clear meaning: “mood not good.”
In many Western cultures, especially American culture, there can be an emphasis on pinpointing and articulating specific emotions (e.g., “I feel frustrated because…”, “I'm anxious about…”). While this is also possible in Chinese, there is a strong cultural preference for subtlety and maintaining social harmony. `心情不好` is the perfect tool for this cultural context. It allows a person to signal their negative emotional state without being confrontational or overly dramatic. It's a “softer,” more general way to express distress. For example, if you're invited to a party but are feeling down, saying “我心情不好,就不去了” (Wǒ xīnqíng bù hǎo, jiù bù qù le - “I'm in a bad mood, so I won't go”) is a perfectly valid and polite excuse. It communicates your state without burdening the host with the specific details of your problem, which aligns with the value of not “making trouble” for others (不给别人添麻烦 - bù gěi biérén tiān máfan). It's a step back from the intensity of `我很难过` (I'm very sad) or `我很生气` (I'm very angry), making it a lower-stakes, everyday expression.
`心情不好` is an extremely common and versatile phrase used in daily life.