Frequency response is a measure of a system's output in dB versus the output in Hz for a given stimulus. In case of a loudspeaker, the sound pressure is measured as a function of frequency. Pink noise is commonly used as a measurement signal.
A frequency response diagram can be taken to show the tonal balance of a system. An ideal loudspeaker has a "flat" frequency response, meaning equal output for any frequency between 20 Hz and 20 kHz, which corresponds to a neutral tonal balance.