Just like Call to Action (CTA) and Color THEORY, Visual Weight is part of the fundamental knowledge a UI designer should have. Visual weight means giving a design element “power” to STAND out and get user’s attention.
Not all ELEMENTS have the same amount of “heaviness”. For example, CTA elements are more important than a simple label or an abstract image. Giving higher levels of “heavy” to certain elements are usually achieved by USING contrast.
This isn’t necessarily a contrast of colors; it can be contrast of placement or size, too. A CTA button might look larger (or heavier) simply because it is designed to take up more space than SURROUNDING elements. This puts more visual weight (more importance) on the CTA button.