Sensitivity, in the realm of headphones and audio, refers to the sound pressure level a headphone can produce when driven by a unit electrical signal (typically 1 milliwatt of power or 1 volt of voltage), measured in decibels per milliwatt (dB/mW) or decibels per volt (dB/V). Simply put, it measures how "easy to drive" a headphone is—at the same volume, the higher the sensitivity, the less power it requires, and even a phone or a small player can easily drive it to a sufficiently loud level.
Its significance for listening experience and purchasing decisions lies in the fact that high-sensitivity headphones (typically >100dB) are better suited for portable devices like phones and tablets, delivering full volume right out of the jack without the muffled sound or lack of dynamics associated with insufficient power. Low-sensitivity headphones (<95dB), on the other hand, often require a dedicated headphone amplifier or a high-power player to perform at their best; otherwise, the sound may come across as thin and lacking in detail. Additionally, high-sensitivity headphones are more prone to exposing background noise, demanding a cleaner source, so trade-offs should be considered when making a choice.
Practical advice: If you primarily use your phone for direct playback, prioritize headphones with sensitivity above 100dB/mW and relatively low impedance. If you already own a professional player, there's no need to chase high sensitivity excessively—focus instead on tonal character and matching with your amplifier's driving capability.