I assume that you are talking about the Proposition 13 that is very famous -- the one that was passed back in 1978 -- not the one that is on the ballot now that has to do with seismic retrofitting.
Proposition 13 was mainly intended to reduce property taxes in California. There are a number of technical details as to exactly how this was to be done, but the basic idea was that property taxes could not exceed 1% of the value of the property. In addition, the assessed value of a property was, for the most part, not allowed to increase by more than 2% each year.
By lowering the rates and by ensuring that the value of the property could not be increased by assessors, Prop 13 reduced the amount of property tax that property owners had to pay.