Electrical impedance is like resistance but with AC electrical current (flows back and forth, which is what creates vibrations in headphones to generate sound) instead of DC current (flows in one direction). Electrical impedance can be different for lower frequencies (e.g. headphone bass) and higher ones (e.g. headphone treble).

Headphone/earphone makers account for this when designing gear. If an amplifier has a high output impedance, it adds impedance, potentially skewing the ratio and, thus, the sound. That’s why you generally want low amplifier output impedance, though some headphones can sound better with high.