The perfect resistance is where the pressure from the espresso machine's pump only just wins the battle when confronted with the resistance of a tightly packed finely ground coffee filled portafilter basket. The closer you can manipulate the elements to create the perfect resistance, the better your chance of extracting 'the god shot'.
On one side of the perfect resistance is a powerful volumetric pump set to 9-10 bars of pressure when extracting. On the other side is the fineness of the grind, the amount of ground coffee, the compactness of the two in the portafilter basket and the pressure you are able to exert on the ground coffee as you engage the portafilter with the espresso machine.
You will know that you have the perfect resistance when an extra half a teaspoon of ground coffee causes the pump to stall and the lights on the espresso machine to flash. The coffee extraction from the perfect resistance will be delayed 3-5 seconds after pushing the button, start by dripping slowly in a thick honey-like density, build to a mouse-tail look as the espresso extraction curls under the portafilter spout and thins as it falls. Here's one I prepared earlier -