What is the physical meaning of resistance and power ratings of a loudspeaker?
The specified impedance is the sort-of average over the relevant frequency range for the speaker. Googling for "speaker impedance" shows a few graphs. The impedance consists of an ohmic part and a much more complicated part. The ohmic part is (hence) much lower than the rated impedance.

The power rating is the total amount of electrical power you are allowed to feed the speaker without destroying it (but there might be other limits, like the amount of power in certain frequency bands). It is certainly not the energy in the sound waves produced: a speaker has a very low efficiency, Wikipedia cites 0.5 .. 20%.