What you need to know
Headings provide a quick outline of the page. People scan them visually, and screen-reader users can jump between them. A consistent hierarchy also helps writers and developers see whether the content is organized clearly.
How to apply it
Start with a descriptive h1 for the page topic, use h2 for major sections, and use h3 for a subsection inside an h2 section. Keep headings in the order the content is organized.
Common mistake to avoid
Do not pick h2 or h3 simply because the font looks smaller. Use CSS for appearance, and do not skip levels just to get a desired size.