Is speaker impedance, which is measured in ohms, important? Plus, do you need to match speaker and receiver ohm ratings?

You probably know that to transfer the most energy between a source and a load their impedance needs to match. That’s why a ham radio transmitter needs a 50 ohm antenna (at least, usually).

For that matter, what's an ohm? While impedance technically does matter, it's not quite as cut and dry as saying "speakers with X impedance should only be connected to receivers with Y impedance." ...