These are series termination resistors which are recommended if the SPI communication lines are long. This means putting 100R resistors as close to the driving source as possible. Thus we need to put resistors in CLK, MOSI and CS\ lines close to the host. At the slave we would use a resistor in MISO. These resistors series terminate the lines to reduce or eliminate undershoot and overshoot at the receiving end. Care must be taken with multiple SPI slaves though, series terminating is best with point to point connections.