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.