http://en.wikipedia.org/wiki/Electrical_impedance
http://en.wikipedia.org/wiki/Output_impedance
http://en.wikipedia.org/wiki/Input_impedance
http://www.williamson-labs.com/
As you can see in the above links, there's lot's of theory to get overwhelmed by, but there are also many tutorials available... A search will give you dozens to look through.
To keep it simple, I think of output impedance as just a resistor in line with the output lead of a generator that by itself could deliver infinite current without a change in voltage.
Let's say the generator produces 75 volts, no matter how much current flows in the circuit. And lets also say that the 'Output Resistance (Impedance)' is 75 ohms.
Q1. How much current would flow if nothing was connected to the output terminals? A. zero, the load resistance or impedance is virtually infinite so no current can flow.
Q2. How much current would flow if a short circuit was connected to the output terminals? A. 1 amp, the load resistance or impedance is virtually zero ohms in the case of a short circuit so the current is only limited by the resistance inside the generating devise. Ohms Law says, Voltage divided by Resistance equals Current so in this case 75 volts / 75 ohms = 1 amp.
Q3. How much power will be dissipated outside the generating devise in the case of Q1 & Q2? A. Zero watts. Because If either voltage or current is zero across or through a resistor, Ohms/Watts Law shows that no power is dissipated.
Q4. How much power will be dissipated in a load resistance of 75 ohms when connected to the terminals of the generating device?
If you like, I'll let you answer that...