The concept of energy vs conditioning is relatively simple.
Take your fighter as a car: Imagine your current energy value as the size of your car's gas tank and imagine conditioning as the efficiency of the engine.
If you have a low level of conditioning then you will burn through each litre of gas more quickly. The smaller your gas tank was in the first place (your energy value), the less gas you have to burn in the first place and the quicker you will run out of gas completely.
Note: conditioning can go down as well as up. If you take a session off, your fighter will rest and recover energy but will take a small hit to their conditioning levels from lack of training.