This means that CSS properties set on parent elements are copied down (cascade) to the child elements.
Some properties are prevented from cascading by default values that are set on some elements. An example of this would be the border set on images if they are inside an anchor tag or the underline on the text inside an anchor tag. These are not cascaded down to them but are default values of the elements.
When a default value changes, it will cascade down into child elements from that point.
Specificity is the term that explains how the browser determines which styles to apply first, second, third, and so on.
The basic idea is that default styles are applied first, then tag styles, then class styles, and followed by the id styles last.
However, the location of the style also applies. Styles in an external sheet, inside a style tag, or inline inside a style attribute have an effect on the order.
Pseudo-elements, pseudo-classes, and the number of each type of selector also has an impact.