Measured in Ohms (and also represented by the symbol "Ω"), impedance relates to a device's resistance to the current being pushed through by the audio source. The reason why this value matters is that headphones with lower impedance typically require less power to deliver optimal audio levels. This works quite well with the mobile devices that consumers mostly use to listen to music (or podcasts, or audiobooks) these days.
Higher-impedance headphones demand more power to deliver their full audio reproduction potential – a reason why they are better paired with sturdy amplification. Headphones used in the audio industry often (but not always) have a high impedance value, as they're matched with powerful amps and may fulfill specific requirements of professional studio systems.
Keep in mind that high impedance doesn't necessarily imply superior (nor inferior) audio quality. We recommend that you always make use of our search features to compare more features and choose the headphones that better cater to your needs.