Question re matching pre and power amp impedance..

I apologize if this is a very basic question but I was hoping I could get someone to explain (in layman's terms)what one ought to look for when matching the output impedance of a preamp with the input impedance of a power amp, in case there is a rule of thumb. Would appreciate an explanation of the basic concepts of what to look for and why (again for the layman please :-)). I seem to remember reading somewhere that you should look for a power amp that has an input impedance at least 10X the output impedance of the preamp it is matched to, for example. I am not sure if this is correct in the first place, but if it is the right rule of thumb, I would love to understand why (again the basic concept behind this). Thx very much.