If a FPGA or CPLD needs to be coupled to a microcontroller often a fast interface is required. Many controllers do not provide an external address/data bus. Even if, this would require many wires and pins at both parts. Alternatively we can use one of the standard serial peripherals such as
typical max. baud rate
often: 400 kbit/s
sometimes: 1 Mbit/s
The given core is a SPI slave which receives the SCLK, MOSI, MISO and SSEL signals from the SPI master (microcontroller). The master starts a transaction by sending a command followed by data. The command can be write, read, set, clear or toggle. It is decoded in the core and wishbone transactions on the master port are generated. Your design will be connected to this master port as wb slave(s) and now can be controlled by your controller.