The heading fits at the bottom of page three, but its explanation begins on page four. Adding blank paragraphs above it looks like a quick repair. Edit an earlier sentence, though, and the empty space moves with the text.
Give the heading a pagination rule
In desktop Word for Windows, place the cursor in the heading paragraph. Open the Paragraph dialog from Home, then the Line and Page Breaks tab. Enable Keep with next. This keeps that paragraph with the following one when Word lays out the pages.
Do not confuse it with Keep lines together, which concerns the lines inside one paragraph. Page break before serves another purpose: use it when a chapter must always start on a new page, even if space remains above it.
Test with a small edit
Try the change in a copy. Add a sentence earlier in the document, then remove it. Check whether the heading and the start of its explanation still travel together. Turn on formatting marks if they do not: an empty paragraph between them may be the paragraph the heading is actually linked to.
Avoid applying Keep with next to every ordinary paragraph. A long chain of linked paragraphs can create large gaps as Word tries to keep them together. If many headings need the same rule, adjust their style after checking which text uses it.
Finally, inspect chapter openings and the exported PDF. A little space at the bottom of a page can be the correct result: it is often easier to read than a heading separated from its explanation.