Serial Peripheral Interface(SPI) is a serial communication protocol. As its name suggests, it is used for communication between a master device and its peripherals. It is a four line interface.
SCK - Serial clock
Two lines for data - one from master to slave (MOSI), and one the other way (MISO)
CS bar - Chip select
SPI is actually not that strictly enforced, some devices might add in their own modifications. It is generally faster than other serial communication protocols like UART and I2C.