Plain English Meaning
It is how hot the gas is at a point inside the turbine part of a jet or turboprop engine. The pilot watches this number to make sure the engine is not getting too hot.
Full Definition
In a turbine engine, the temperature of the gas measured between two stages of the turbine section, typically between the high-pressure and low-pressure turbine wheels. ITT is displayed on a cockpit gauge and is one of the primary engine limits the pilot must monitor during start, takeoff, and high-power operations.