\frenchspacing is a Plain TeX macro which switches off extra space after punctuation.
By default (and to implement the American custom in typography) the Plain TeX format ascribes more stretchability and shrinkability to spaces which follow punctuation characters. The Plain TeX macro
\frenchspacing cancels this mechanism. There is also
\nonfrenchspacing which reinstores it. These macros and the same default are taken over as is in LaTeX.