Think of your audio system like a car. Your car’s engine needs to provide a certain amount of power to keep it moving down the road, and more power to overcome its inertia and start moving, or accelerate.

Any time energy is transferred between parts in your car, that implies friction. The total friction of the parts in your car is analogous to electrical resistance. But it’s not the only thing the car has to overcome. There’s things like the wind, the terrain, the gear you’re in, etc.

The total of all these components, weighted against each other, is the total factor by which the effort of your engine is being held back. Its impedance, in other words.

If you shift a car into a high gear without letting it gain enough momentum, it’ll bog; the efficiency of the whole system will go down. You may not completely come to a stop, and you may recover eventually if the engine is simply allowed to work a bit too hard for a bit, but you don’t really want to do it this way.

Similarly, headphones with high electrical impedance *can* run on output sources not rated to handle this impedance. But their performance will be reduced, they won’t sound as good, because the signal is too weak, everything has to stumble around to keep up. The typical solution to this is to raise the volume, but then you’re left with reduced dynamic range. In general, too little power to a pair of headphones makes them sound dull, dark, or muddy.

Modern headphones are generally pretty low-impedance. This is because the specifics of their design is simply more efficient. Unless a pair of headphones specifically tells you its impedance, you don’t generally need to worry about it.