I2S or "I squared S" is a short range sustem for sending information between digital audio ICs. Such as between a DSP and an Analog to digital converter. It consists of 3 clock lines (MCLK, SCLK and LECK though sometimes these names are different) and one or more 2 channel audio lines (SDATA) 

It is common to use I2S at 24 to 32 bit depths and high clock rates. 384K is common. Even 768K at times. 

The writer has not included material on it in this collection as there is a lot of information available elsewhere.