Cascading Style Sheets (CSS) -- What does the Cascading part mean?

The basic principle is this: When two or more CSS selectors select the same HTML element, the most specific selector has the highest priority.

This means a higher priority style property will override a lower priority one.
For example, a background-color from a more specific selector (higher-priority) will override a background-color from a less specific selector (lower-priority).
The official terminology is that a more specific selector has higher specificity than a less specific one.

See the source code for more information and why the blocks are the colors they are.

The Element Selector and Generic Class Selector both select this block.

The Element Selector, the Generic Class Selector, and the Paragraph-specific Class Selector all select this block.

All 4 Selectors select this block. The ID Selector wins being the most specific.
Each ID must be unique per page, but a style class can be used multiple times.
Therefore, class selectors are more versatile than ID selectors.

Styles applied as the value of the HTML style attribute (inline-styles) are the most specific (highest specificity).

Inline Styles will override styles applied by ANY Selector. This is the lowest level in the cascade.