The SPI bus is a full-duplex, single-master, multi-slave, synchronous serial data bus and, as the I2C bus, it's used for on-board connection of sensor chips with the main CPU. This bus require at least (apart the GND signal) three wires plus one chip select signal per slave, this line is typically called Slave Select (SS) or Chip Select (CS) and usually it's active low (that is the master must set it to 0 to enable the desired slave chip).
Some terms need to be explained here:
Full-duplex:It means transmitting and receiving are at the same time on the bus.
Synchronous: It means that the clock is sent along with the data (in this case, it is the master that provides the clock).
Single-master and multi-slave: It mean that on the bus, there is one master only that directs the communication, while more than one slave can be connected on the bus.
Serial data:It means data is transmitted one bit at a time over the bus.
The communication starts when the...