A widow is the final word in a block of text that falls to its own line. Especially if the widow is only a few characters long, she can get lonely. wp-Typography will try to protect widows by bringing them company from the previous line.
There is danger that the widow’s company will leave the previous line with less than optimal word spacing. The risk is less if your text is left-aligned, but if it is justified, tread carefully. The protection of widows is completely customizable in the administrative options.