I came across this 7805 voltage regulator. In the datasheet is mentioned the current vs voltage graph. What is Input to output diferrential volatge?
It is literally the difference between the input voltage and the output voltage. If the input is 9V and the output is 5V then the differential is 4V.
The reason the graph doesn't go all the way down to 0V is due to the dropout of the regulator.
It is probably the difference between the input of your 7805 and the output of your 7805. The output of the 7805 is always 5V. The input of the 7805 can vary between roughly 8V - 35V.
Looking too this graph u can see a few things, as the minimum drop off voltage (looks like 2,5V - 3 V), what the best input if for the most peak current (would be like 13V - 15V for max peak current), and you can see what the maximum input voltage is. (maximum input voltage is around 35V).