Before we explain anything, you should know that the only group that opposed this measure withdrew its opposition and switched from "opposed" to "neutral."^[4] So at this point, no group opposes the measure, and it is very widely supported. The SF Chronicle even says the measure is so complicated in its language that Prop 52's biggest challenge is getting people to understand it enough to not skip over it or vote No.

California's Medi-Cal (the state version of Medicaid), which is essentially medical services for low-income residents, is funded by the state and matched by the federal government. Whenever tax-payers put in a $1, the federal government matches it with about $1.

Well, in 2009, the state legislature was like 😏  and decided to tax private hospitals instead of all Californians. They took most of that $4.6 billion and put it into Medi-Cal, thus triggering the federal matching grants to the tune of $4.4 billion, making a net ~$3B. They then gave that back to the hospitals, private and public, (although not all private hospitals fully recovered the tax fee). To put it loosely, California is making money (~$3B) through this scheme (paid for by US taxpayers), and Prop 52 codifies this into the state constitution.

Prop 52 stipulates that legislators cannot divert funds away from Medi-Cal, thus ensuring that Medi-Cal and hospitals get the money.^[5]

The people who wrote this proposition, the California Hospital Association (those who get taxed), didn't want legislators wrangling with the terms.^[5] It works well as is, they contend. The state already takes about a quarter of it for the General Fund, but if sides start squabbling, they might take more, or the tax might not get renewed. The Hospital Association says this money is crucial.^[..] Hence, they want it protected in the constitution (but with the ability for it to be repealed with a 2/3 vote).^[3]