How loud a speaker plays for a given input power, quoted in dB (e.g. 87dB/2.83V/1m) — higher sensitivity needs less amplifier power.
Sensitivity tells you how efficiently a speaker turns watts into volume. An 87dB speaker is fairly typical; a 91dB speaker is meaningfully more efficient and will play the same loudness from roughly half the power. High-sensitivity designs (95dB+, often horns) can be driven to satisfying levels by low-powered valve amps — which is exactly why those pairings exist.
It interacts with impedance and room size. A small, low-sensitivity standmount in a big room with a modest amp will run out of steam; the same amp into an efficient floorstander may have power to spare.