Prop 16 would repeal Prop 209 (1996), which banned giving preference based on race, sex, color, ethnicity, or national origin for public employment, education, and contracting. Put more simply, Prop 16, if passed, would allow public institutions to use affirmative action for hiring, admissions, and contracting.
Affirmative action is intended to correct the effects of disparity and discrimination. Democrats largely support it (and Prop 16) because it can be used to open the door for underrepresented populations that have been oppressed for centuries. Specifically for Prop 209, they point to a study that shows Black people mostly lost out as a result. Republicans tend to argue that we should not be fighting discrimination with more discrimination, and that advantage for one group of people is a disadvantage for others, assuming a zero-sum situation.
Prop 16 is easily the heaviest and most consequential proposition on California's ballot. Whatever you do, please vote informed.
The question posed by Prop 16 is straightforward, but the history of affirmative action is deep and winding. It intertwines with racial injustice, underrepresentation of women, the Civil Rights Act of 1964, and several Supreme Court rulings. For f's sake, just browse the Wikipedia page. Both sides hold deep-seated convictions, point to many studies that support their rationale, and the argue that their solution is more equitable and fairer. For now, we won't attempt to boil down the history. We had one job, we know, but any summary here would lose the nuance of the context of each turning point.
It's important to note that given its complex history, its usage is also complex. Legal affirmative action is widely used today throughout the US. It is used in (some) state and federal government contracting, at some public and private university admissions, and very widely in hiring practices of private companies. Prop 209 bars the public sector in California from using it. UC Berkeley, a public university, cannot use race as a factor in its admissions process, even though Stanford, a private university, does.