Explain the Principles of Typographic Hierarchy to a Complete Beginner

Typographic hierarchy is the most important design principle a non-designer can learn. Without it, all text feels equally important, which means none of it is. With it, the reader knows what to read first, what to skim, and what to refer to. This explains the principle, the three levels, the four tools in the order to use them, and the single most common beginner mistake, with a test to know whether it is working.

I am designing something for the first time [a poster, a document, a slide, or a simple website] and I want to understand typographic hierarchy. I have no design background.

Explain the principles:
1. What typographic hierarchy is: what it does and why it matters for the reader, explained as a functional concept rather than an aesthetic one
2. The three levels: how to think about three levels of typographic hierarchy (primary, secondary, body) and what decisions define each level
3. The four tools: the [four] typographic tools for creating hierarchy (size, weight, colour, and spacing) and the order in which to apply them for maximum effect with minimum complexity
4. The most common beginner mistake: the single most common hierarchy error made by people without a design background, and what it looks like and why it fails
5. A test: how to check whether the hierarchy is working, stated as a question a reader would answer if the hierarchy is successful