Two things will happen if this passes.
(1) Inmates serving time for a nonviolent crime – as defined by the Penal Code – will now be eligible to earn credits for good behavior, educational achievements, and rehabilitation, that can be used to shorten their sentence. The credit system already exists, and state law already limits how much a sentence can be shortened. Prop 57 would now require inmates to serve their full primary sentence – their main term of imprisonment, excluding any extraneous sentencing.

(2) If a county prosecutor decides a juvenile should be tried in adult court, then a judge must confirm it in a separate trial. (Right now, county prosecutors may send a juvenile to adult court without a judge's approval.)

As you can imagine, the media is focused on (1) of this proposition.

On one side, there's a US Supreme Court mandate to California to reduce its prison population.^[..] California has already reduced its prison population by 50,000 since 2009 but is still over capacity by 50,000.^[12] This proposition would create a yet-to-be-defined system where inmates could earn their way out of prison. Some say their release might be inevitable,^[6] so why not incentivize rehabilitiation?

Here's something vague: Because this new credit structure has yet to be written, Prop 57 might end up applying to more than just non-violent offenders.

More vagaries: non-violent is not clearly defined. Rape of an unconscious person, taking hostages, and setting off a bomb with intent to injure are not considered violent crimes, according to the Penal Code 667.5(c). (In judicial parlance, there are three non's: non-violent, non-serious, and non-sexual.) There is no clear demarcation of what a non-violent crime is. In practice, criminals who've committed these more heinous acts are excluded from the credit system now, but it doesn't specify whether that would extend with Prop 57.^[4]