Power rating for speakers

Essentially nothing. These are approximate power ranges the speaker manufacture thinks are proper for the speaker. But amps vary tremendously in their ability to deliver power at different impedance's. Also remember that far more speakers are damaged by too little power than too much. Get the efficiency of the speaker , measured in db for one watt input. Increasing the output 3db requires twice the power. See if the amp you are considering has more than enough to drive the speaker to its full rated output.

Al undoubtably has answered this but I'll try to make it simple. When a speaker is rated at 100 watts it should mean 100 continuous watts. Music is composed of peaks. A 100 watt speaker can tale PEAKS of several times this. If you take a low powered amp and drive it into clipping [beyond its capability] it will eventually start to pass DC current through to the drivers and end by burning them out. so you should err on the high side rather than the low. If you are careful you can use ALMOST any amp with any speaker, but matching power output and speaker requirements make life easier.