A widow, typographically speaking, is a word or part of a hyphenated word that is alone on a line in a paragraph. The text comes to the end of the line and one word or part of a word wraps to the next line like
An orphan is the last word or part of a word of a paragraph that jumps to the top of the next column or the next page.
How best to avoid them? Designers often use tracking and kerning to fix. (If these terms are not familiar to you, good.) How should you fix them? Add or eliminate word. Seriously, add or eliminate words. I track or kern because I usually don’t have the authority to alter content. When I can alter the copy, usually because I wrote it, I alter the copy and you should too.