gtroff reads text, it collects words from the input and fits
as many of them together on one output line as it can. This is known as
gtroff has a filled line, it tries to adjust it.
This means it widens the spacing between words until the text reaches
the right margin (in the default adjustment mode). Extra spaces between
words are preserved, but spaces at the end of lines are ignored. Spaces
at the front of a line cause a break (breaks are explained in
Implicit Line Breaks).
See Manipulating Filling and Adjusting.