Prop 20 has a lot of provisions that make changes to state criminal law, generally making it tougher on individuals who have committed crimes or are on parole.

• Prop 20 allows some theft-related crimes to be classified as felonies, instead of misdemeanors, such as stealing property worth more than $250.  It also creates two new crimes: serial theft and organized retail theft, both of which could be punished as either a misdemeanor or a felony. 
• Prop 20 makes more difficult the process introduced by Prop 57 (2016), which has allowed individuals convicted of non-violent felonies to be considered for release if they've served their primary term. (i.e. served time for the main crime they committed).
• Prop 20 expands for which crimes DNA must be collected, including shoplifting and domestic violence. Currently, DNA is collected for people committed of felonies.

Prop 20 has a lot of details, but generally a Yes vote would increase criminal punishment and make it harder for convicted felons to be considered for parole. A No vote will keep things as is.

Advocates of Prop 20, which include prison guards, police unions, and grocery store chains, make the argument that Prop 57 (2016) was flawed, most notably for not classifying many crimes – such as rape of an unconscious person, domestic abuse, and drive-by shootings – as "violent" offenses. Because Prop 57 failed to define what a  "nonviolent" crime is, it defaults to a crime that's not technically a violent crime according to the penal code. Prop 20 would double the list of crimes that automatically disqualify someone from consideration for release. (See the full list in the legal text, page 18). The Yes side also point out that those released from prison sometimes reoffend, sometimes with serious crimes.

Opponents argue that Prop 20 is a step backwards.

The overall goal in prison reform has been to ease harsh sentences for low-level crimes, reduce the state's overcrowded prison population, and create more opportunities for parole and rehabilitation. Thanks to Prop 47 and 57 , we have reduced our prison population. Prop 20 would go against that trend and increase incarceration, particularly for Black, Latino, and low-income people for low-level crimes. A larger prison population reduces the budget for other programs, they say, and making early release more difficult would reduce the incentive for inmates to demonstrate good behavior. 

Two studies compared California to comparable states to see how Prop 47 (2014), which reclassified low-level drug and theft crimes as misdemeanors, affected crime and arrests. Hilariously, both sides point to the same studies.

Supporters of Prop 20 point to the evidence that theft (larceny and vehicle theft) has increased moderately after Prop 47, although it's not clear how much of that effect was caused by Prop 47.

Opponents of Prop 20 point to the conclusions that Prop 47 reduced violent crime, prison populations, and recidivism (re-arrest and re-conviction).