To understand CombinedFT, you absolutely must understand fuel trims in general first. Please read through that tutorial first if you have any questions at all on what fuel trims are or how the ECU works with them.
CombinedFT simply adds the current short term fuel trim (STFT) to the currently selected long term fuel trim (SelectedLTFT). The ECU selects different long term fuel trim values as driving conditions (primarily airflow) change. Low airflow typically seen at idle selects the LTFTLo value. Light cruise conditions typically select LTFTMid. Mild cruise acceleration conditions typically select LTFTHi. On a 2G DSM, LTFTHi is actually never selected unless you have manually adjusted the LTFTSelect table in direct access for your ECU.
So CombinedFT is basically showing the combined total of what the ECU is doing with fuel trims at any given moment, regardless of which LTFT is selected at the time. It's really a convenient view of the total compensation the ECU feels is necessary to maintain closed loop operation.