when output impedance is small a relatively large output current can be drawn from a devices output without significant drop in in output voltage.When the output impedance is large a relatively small output current can be drawn from the output of a device before voltage at the output drops substantially.A rule of thumb for efficient signal transfer is to have an output impedance that is at least 1/10 of the loads input impedance to which it is attached.
It turns out to be pretty easy to measure source (or output) impedance. You need only a variable resistor (like a pot) and a volt/ohms meter. Measure the unloaded output voltage from the mystery source. Then attach a variable load (your pot) across the output and increase the load until it is 1/2 the open circuit (unloaded) voltage which you previously measured. Then disconnect the pot and measure the resistance. That resistance is equal to your source impedance. The principle is that you are forming a voltage divider. The "top" resistor in the divider is the mystery source impedance, and the "bottom" resistor is your pot. When the voltage is 1/2, then the two resistors are of equal value.Of course, if you are measuring something at the extreme ends of the spectrum, conventional parts and techniques don't apply. So measuring the source impedance of a guitar pickup or a car battery can't be done using this simple scheme.