For almost every speaker you can buy, you'll find a specification for impedance, measured in ohms (or Ω). But they never explain what impedance means. Fortunately, impedance is kind of like great rock'n'roll. Understanding everything about it is complicated, but you don't need to understand everything about it to "get" it. So for this article, I'll tell you exactly what you need to know about speaker impedance without making you feel like you're taking a graduate-level course at MIT.
It's Like Water
When talking about things like watts and voltage and power, a lot of audio writers use the analogy of water flowing through a pipe. Why? Because it's a great analogy! That's why I'm going to use it here instead of coming up with something more original.
Think of the speaker as a pipe. Think of the audio signal (or, if you prefer, the music) as the water flowing through the pipe. The bigger the pipe, the more easily water can flow through it, and the more water can flow through it. A speaker with a lower impedance is like a bigger pipe. It lets more electrical signal through, and lets it flow more easily.
This is why you'll see an amplifier rated to deliver, say, 100 watts into 8 ohms impedance and maybe 150 or 200 watts into 4 ohms impedance. The lower the impedance, the more easily electricity flows through the speaker.
So does that mean you should buy a speaker with lower impedance? Not at all, because a lot of amplifiers aren't designed to work with 4-ohm speakers. Think back to that pipe carrying the water. You can put a bigger pipe in, but it'll only carry more water if you have a pump powerful enough to provide all that extra water.
Take almost any speaker made today, connect it to almost any amplifier made today, and you'll get more than enough volume for your living room. So what's the advantage of, say, a 4-ohm speaker versus an 8-ohm speaker? None, really. Except one: Low impedance sometimes indicates the amount of fine-tuning the engineers did when they designed the speaker.