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