Explain impedence matching please

Can someone explain to me how to match impedance when considering an amp and a pre? Like, for instance, if a pre has an output impedance of 800 ohms, and the amp has an input impedance of 200K, what does that mean as far as matching?
The general rule of thumb is that the preamp have 1/10 the output impedance with respect to the input impedance of the amplifier. It should not exceed that value at all audio frequencies. In the case you describe above, no worries.