An impulse response is the acoustic 'fingerprint' of a space or device. It is captured by playing a short test signal (sine sweep or actual impulse) and recording the result. The IR contains: direct sound arrival time, early reflection times and levels, frequency response (via FFT), and decay characteristics.
Applications: convolution reverb (applying a room's IR to dry audio), room correction (inverting the room's IR), speaker measurement (capturing the speaker's anechoic response), and guitar cabinet simulation (applying a cabinet's IR to a preamp signal).