What does exactly mean the power specified in a speaker datasheet?
The power is the maximum electrical power the speaker can handle without damage. It has only a rough correlation with acoustic power output: some (perhaps even most) of the electrical power also goes into losses in the speaker coil, magnetic losses, friction losses, and so on.

If the speaker is rated for a higher power than your amplifier, that's good. It means it can take all the amplifier can dish out without damage. If the speaker is rated for less, then your amplifier at full volume may damage the speaker, and usually that's bad.

Beware that cheap speakers tend to use meaningless methods to arrive at their power rating, like the peak power they can take at the very peak of a cycle, instead of the average power of the AC being fed to them. When you see those $5 "100W" computer speakers that sound too good (the price, not the quality) to be true, that's what's going on. Your speaker sounds legit, however.

Further reading: audio power at Wikipedia.