Couple of peripherals like SPI and UART derive their clock from the fPCLK. So what clock is PCLK and what frequency is fPCLK?
PCLKx is the clock of the corresponding APB peripheral X. For example:
Note: Similarly, HCLKx is the clock of the corresponding AHB peripheral X. For example:
So, when SPI2 says it derives its clock for fPCLK, what it means is the clock of its APB bus. In STM32F407, SPI2 is on APB1. So this makes its fPCLK fPCLK1.