SPI secondary device support allows a board to act as a device that's controlled by a SPI main. This is handy if you need a device to look like a SPI peripheral, or if you want to control multiple devices from one main device. Each device can act as a SPI secondary and the main can send and receive data with them as necessary.
In MicroPython SPI secondary support is still in development for some boards. The ESP8266 port in particular does not currently support SPI secondary mode. This guide won't go into detail about SPI secondary support, instead check your board's documentation to see if it has SPI secondary support and learn about its usage.