If a paragraph begins at the bottom of a page, just before a page break, so that only the first line of the paragraph appears on that page, and the rest of the paragraph resumes after the page break on a new page, the isolated line is called a "widow." Widows appear unprofessional. Place page breaks so that you have at least two, and ideally three, lines of a paragraph at the bottom of a page.
Isolated lines can also occur at the tops of pages. If there is a page break just before the last line of a paragraph, that last line will appear as an "orphan" on its own at the top of the new page. Eliminate orphans as well as widows to make your document appear more professional.
A subheading located just before a page break will appear on its own at the bottom of a page. Isolated subheadings are even more noticeable than widows and orphans and should be eliminated.
Most popular word processors have a "widow and orphan control" feature. Turn this on for automatic widow and orphan control. Although these features are relatively reliable, always proofread manually to make sure they are functioning correctly.