If the voltage is on the x-axis and the current is on the y-axis,
then the slope of the curve represents the resistance of the
device that the voltage is being applied to.
If the graph is NOT a straight line, then it gets more interesting.
It shows that the resistance of the device or component is not
constant, but depends on the voltage applied to it. A diode is
a good example of that. The voltage/current graph of a diode
has sudden changes of slope ... nothing gentle about it.