Before WWII, significant numbers of women worked in light industry, such as clothing manufacture, but very few in heavy industry, such as airplane production. As large numbers of working aged men were drafted or enlisted, there was a shortage of labor in plant, and in particular, war industry.
Women were recruited to "man" the assembly lines. This was a dramatic cultural shift. The image of Rosie the Riveter was coined to both recognize those women contributing to the war effort and to rally support.
She appeared in numerous posters and a film was made as well.
The iconic image of the flexed muscle is still widely seen, currently on refrigerator magnets, as a symbol of female power. However many younger folks in the U.S. have no idea who Rosie was or why her role was significant.
Ironically, many women who worked in factories were displaced as men returned from war. The fact that women had successfully done jobs that previously were viewed only as "man's work" would eventually help women as they sought equal opportunity in the workforce, albeit decades later.
See this article.
SUPPLEMENT: Below is a poster with a female war effort worker who is usually identified as Rosie.