Definition: Frequency response is a specification that indicates how evenly a speaker or electronic component reproduces sound over the range of human hearing. A frequency response specification includes the frequency range (usually 20Hz - 20kHz) and the amount that the specification deviates from perfectly flat response, measured in decibels.
Examples:
The frequency response of my speakers is 20Hz - 20kHz, +0.5dB, -2dB.