The Arduino SPI reference page has a good explanation:
Note about Slave Select (SS) pin on AVR based boards
All AVR based boards have an SS pin that is useful when they act as a slave controlled by an external master. Since this library supports only master mode, this pin should be set always as OUTPUT otherwise the SPI interface could be put automatically into slave mode by hardware, rendering the library inoperative.
It is, however, possible to use any pin as the Slave Select (SS) for the devices. For example, the Arduino Ethernet shield uses pin 4 to control the SPI connection to the on-board SD card, and pin 10 to control the connection to the Ethernet controller.
So the SS pin on the Arduino board (which is the SPI master in this case) is simply being used as CS (chip select) for the CC1101, which is the SPI slave. You could use any pin on the Arduino board for this. However, since the use of SPI already puts limitations on the Arduino board's SS pin, it's convenient to use it for CS.