Impedance, in simple terms, is the headphone's resistance to the flow of electrical current, measured in ohms (Ω). It is not a fixed resistance but rather an "AC resistance" that varies with frequency—essentially describing how much power the headphone "consumes" at a given frequency and how much voltage it needs to be driven. Low-impedance headphones (e.g., 16–32Ω) are easy to "feed," and can produce sound when driven directly by a phone; high-impedance headphones (e.g., 150–600Ω) are like speakers with a large appetite, requiring stronger front-end power.
Impedance directly affects sound quality and pairing. Low-impedance headphones have high sensitivity, but they are easily affected by the output impedance of the player, which may cause background noise or a harsh sound. High-impedance headphones generally deliver more stable sound and better control, but if the driving power is insufficient, issues such as weak sound, loose bass, and piercing treble may arise. When choosing, if you plan to drive them directly with a phone or laptop, prioritize low impedance; if you already have a desktop amplifier or a professional player, high-impedance headphones often unlock their full potential.
Practical advice: don't just look at the impedance number—always consider it together with "sensitivity" (dB/mW). When sensitivity is high, low-impedance headphones are easier to drive; when sensitivity is low, even low impedance may be difficult to drive. During a trial listen, set your everyday device to 60% volume—if the sound is clear, powerful, and free of distortion, it's the right choice.