Impedance in layman’s terms is how difficult it is to drive a set of headphones, rated in ohm. The more ohm it’s rated for, the more volume you need to crank to get the same level of sound. IEMs/earbuds tend to be around 16-40 ohm, perfectly easy to drive with anything. Headphones tend to start from 32 up to 600 ohm. A quick & dirty guide would be up to 70-80 ohm, you can use your phone, laptop, cheapo dedicated audio player (DAP). Beyond that you’d want a dedicated amplifier (expesive DAPs tend to have powerful amp in it).

That’s not all though, because there’s also sensitivity, measured in dB SPL/mW. Sometimes db SPL/mV which I honestly still don’t understand that much. But, the rule of thumb is; the smaller the number in dB SPL/mW, the harder it is too drive. With the “ideal” sensitivity at 100 dB SPL/mW. Can go +/- 10. Sometimes -20.

High impedance headphones were originated from around 1970s when manufacturers can finally make good sounding headphones, but the high price meant they were only viable for studio monitoring application. There are many entwined audio gears in a studio that by the time it reaches the headphones plug, there was gonna be a lot of static noise in the headphones. High impedance will filter out that noise.

The high impedance = good sounding headphones paradigm today still pretty much comes from that 1970s principle. To filter out static noises that might be in the DAC and amplifier. But these days, you can get good sounding headphones with a much lower impedance.