SPI, or Serial Peripheral Interface, is a protocol on many of Atmel's AVR ICs. It allows high-speed synschronous data transfer between the IC and peripheral devices or between other AVRs.
There are four pins used for that, MOSI/MISO/CLK/SS (on an Uno, those are pins 10, 11, 12, and 13). Fun little protocol to play with actually. 