SSP is a state-funded program for people who receive SSI. To put it simply, California adds a certain amount of money to the monthly amount SSI recipients receive from the federal government. This amount can change from year to year. In 2019, the maximum SSP was $160 per month for an individual.
The theory behind SSP is that California has a high cost of living relative to other states. As a result, it wouldn’t make sense for California residents to receive the same amount of SSI that residents in, say, North Dakota receive.