A correct hierarchy has exactly one h1 for the page’s main title, with h2s for major sections and h3s nested under the relevant h2, without skipping levels (jumping from an h2 straight to an h4, for example). Screen readers let users pull up a list of all headings on a page and jump directly to any one of them, which only works well if the hierarchy is logical.
Headings are frequently misused purely for visual styling, such as making a piece of bold text an h3 because it looks right, rather than because it is structurally a heading, which breaks this navigation shortcut for screen reader users.