Die junction temperature is the temperature of the chip inside the package. Many CPU:s have a maximum at 90 - 125 - 150 °C depending on type of chip, exact number can be found in the datasheet for the chip in question.
From that one has to calculate the amount of heat transfered from the die to the case which is given in the datasheet, in °C/W. Next is the case-to-heatsink with same units. And then finally from heatsink to ambient (air) also in the same unit.
