Putting them together, the literal meaning is “not speak, and yet understand.” This beautifully illustrates the core concept: a truth so clear that it can be grasped without any verbal explanation.
不言而喻 is more than just a phrase; it's a window into Chinese culture, which is often described as a “high-context” culture. This means that communication relies heavily on shared context, non-verbal cues, and mutual understanding, rather than explicit verbal instructions.
This idiom is common in both written and spoken Chinese, though it leans towards the formal side. It's not slang, but a standard, educated expression.