"SPI" stands for "serial peripheral interface. It requires 1 clock, two data lines, and one "slave select" per chip being controlled. There is one "master" and 1 or more slaves. So the total number of wires is 3+N, where N = number of slaves. MOSI = master data output, slave input. MISO = master data input, slave output. There are different selection modes, but typically a slave is active when SS = low, or sometimes on the falling edge.