Synopsis, one of:
The first declaration causes LaTeX to treat spacing between sentences in the same way as spacing between words in the middle of a sentence. The second causes spacing between sentences to stretch or shrink more (see \spacefactor); this is the default.
Some typographic traditions, including English, prefer to adjust the
space between sentences (or spaces following a question mark,
exclamation point, comma, or colon) more than the space between words
that are in the middle of a sentence. Declaring
(the command is from Plain TeX) switches to the tradition that all
spaces are treated equally.