Well, it ensures that male and female employees who perform "substantially similar" work are paid equal wages. This language is broader than the "equal work" language in the prior law. This is true even if they have different job titles or work in different offices of the same employer. There is also an anti-retaliation component to the law which allows co-workers to discuss their wages with each other without fear of punishment by the employer. Only merit, seniority, quantity/quality of production, or a "bona fide" factor other than sex that is a legitimate business necessity are allowable explanations for wage differences between male and female employees.
All businesses (public and private) must comply with the law. However, keep in mind that the business must have both male and female employees doing the same or similar work. For example, most nurses are women. If your company has only women who are nurses, then there is no way to apply the law because there would be no male nurse's wages to compare with the female nurses. Of course, this "loophole" (for lack of a better term) existed in the prior equal pay law.
With this law, California remains at the forefront of employee protection.