Moore's law is a rule that predicts what?

It preditcs that computers of the future will be more powerfull... and they will quickly become less expensive as newer more powerful computers will be built,

This can be further explained by the following example:

Your state of the art powerful rig (computer) will loose its value to less than its half in about 3-4 years time...

i hope you get the picture and understand moore's prediction...

best of luck!