Since the odds are not great for finding a set of words, for every
output line, which fit nicely on a line without inserting excessive
amounts of space between words,
gtroff hyphenates words so that
it can justify lines without inserting too much space between words. It
uses an internal hyphenation algorithm (a simplified version of the
algorithm used within TeX) to indicate which words can be hyphenated
and how to do so. When a word is hyphenated, the first part of the word
is added to the current filled line being output (with an attached
hyphen), and the other portion is added to the next line to be filled.
See Manipulating Hyphenation.