The implication is that beauty, or physical appearance, is unimportant.
We can rephrase this: “Not every cloud brings rain.” And that’s true. Sometimes there are many clouds in the sky, but it doesn’t rain. Don’t judge things by appearances.
The attractive exterior of something is not a good indicator of its real nature. It may look valuable, but not be valuable.
The way something or someone looks from outside may give us the wrong impression.
Different people see beauty in different ways. What one person finds beautiful may not appeal to another person.
Physical beauty is superficial. A pleasing exterior is no guide to a person’s interior or character.
Good character and behaviour are more important than good looks.
Some rivers have rough surfaces with waves. That’s usually because the water is shallow and there are rocks near the surface. But deep rivers have no rocks near the surface and the water is smooth and still. “Still waters run deep” means that people who are calm and tranquil on the outside, often have a strong, “deep” personality.
Things may look like one thing but be another thing. This saying is often applied to situations, not just things or people.
You cannot judge what something or someone is like by looking at the exterior.
(published on englishclub.com)