Mildred Pierce is a novel of post-Depression California. Herbert Pierce lost pretty much everything in the stock market crash of 1929, and he never really recovered, unwilling to take any real sort of job, preferring to fool around with a woman who isn't his wife. In 1931, at the beginning of the novel, Bert's wife, Mildred, is finally fed up and kicks him out.
The novel follows her ups and downs and trials and tribulations for the next decade or so. She's a prideful creature, but does what she has to, sinking so low as to become a waitress in order to keep her house and support her two daughters, Ray (as the ridiculously named Moire is known) and Veda. A knack for pie-making and some good business sense eventually allow Mildred to achieve some success: she opens her own business, moving with the times in a way Bert can't (taking advantage of the opportunities that present themselves along the way, such as the end of Prohibition, while Bert is stuck in the past).
Bert remains a presence in Mildred's life, but there are also other men. Her situation now defines her. As a friend explains: "From now on you're fast." Mildred shows a bit of restraint (and usually pays for it when she doesn't), but does get involved with a few men, notably Monty Beragon -- a higher-class Bert (for whom Mildred is never quite classy enough).
The real focus of Mildred's life -- and the destructive core of the novel -- is dear little Veda. Only eleven when the book begins, she's already "something to look at twice".