SCLK – Main clock synchronizing SPI on both devices. Generated by the Master to all the slaves
MOSI – Master Output and Slave Input in which data is sent from the master to the slave on each clock edge
MISO – Master Input and Slave Output in which data is sent from the slave to the master on each clock edge
SS – Slave select, often called CS (Chip Select) or CSn. This line selects the current active slave. It could be Active Low or Active high (refer to the slave datasheet)