What is still waters run deep meaning?

"Still waters run deep" is a saying that means a quiet or reserved person may have depth and complexity beneath the surface. It suggests that people who are calm and quiet on the outside may have strong emotions, intelligence, or hidden talents that are not immediately apparent. It implies that appearances can be deceiving and that one should not underestimate someone based solely on their outward demeanor. This phrase is often used to emphasize the importance of not judging or underestimating others based on superficial characteristics.