It lowered property taxes from 3% to 1%. But it also changed the system whereby properties get reassessed for tax purposes. The tax bill is based on 1% of your property value. The ordinary system is that county assessors will do a new appraisal for each property periodically, like once every five years. Under Prop 13, properties aren't reassessed until they are sold to a new owner. So if your house was appraised at $70,000 back in 1978 when Prop 13 was passed, and you never sold it, then you're still paying 1% of $70,000 ($700) in taxes every year even though your house is now worth $1.3 million. Furthermore, if you die and leave your property to your kids, the property will not be reassessed then either. So if you keep the property in your family, you and your progeny will pay a pittance in property tax for all eternity.
All this means that people in CA have a strong incentive to never sell their houses. Even when they move, they will often keep the property and rent it out rather than sell it. That means low supply in the housing market, which in turn means high prices.