series adds ie: 2x 8ohm in series is 16ohms.
parallel divides ie: 2x 8ohm in parallel is 4ohm. 4x 8ohm in parallel is 2ohm.

amplifier ratings: don't get me started..

amps are normally rated for ~x watts at 8ohms, ~2x watts at 4ohms, etc.. by rating their amps at a lower impedance, they can give the impression that it is more powerful than it really is.

Think of it this way, the higher the impedance, the harder the amp has to work to get the same output level. The lower the impedance, the more current the amp has to supply.

Amps generally don't like to drive low impedance loads, normally 4 ohms is as low at the pro-audio amps used to go, although you are seeing more amps these days that are rated down to 2ohms. I don't think I've ever seen one that was rated down to 1ohm, except possibly for the Mcintosh amps..

speakers are AC.

There is an art to properly matching speakers and amplifiers..

In the pro audio world, you generally want to oversize the amp by 30-50% or so, but it depends a lot on the application and the speaker. You want to be able to drive your speakers close to their limits, without blowing them up, and you want it to sound good while you do so.

You want to allow for some amplifier headroom, to prevent clipping, and you want to avoid sending the speaker cones flying across the room, melting the voice coils, or blowing up the crossover (if you are using passive crossovers).