IA and IB are the two interrupt pins. The interrupt pins can be configured to go high or low when an input either changes state or goes to a preset state. You can find out more about how they work in the datasheet for the MCP23017 on page 24. In the datasheet the pins are listed as INTA and INTB.
Our code libraries for the IO Pi contain all of the functions needed to configure and use the interrupt logic built into the MCP23017 IO chips.