Can't understand concept 'stiff source'
The other answers are good, but you might find a different approach better. Start with a voltage source and a load. The voltage source will have an output impedance. The circuit will look like 







simulate this circuit &ndash; Schematic created using CircuitLab

Let's say that Rout is large. Then changes to the load resistor will cause changes to VLoad. The source (measured at the load resistor) is "elastic". It will vary with changes to the load.

Now let's say that Rout is very small. Then changes to RLoad will have little effect on VLoad. Using the image of the load on the end of a beam, the source is now said to be "stiff". It does not change with changes in load.