(1) In any linear system or device, the output signal is a function of the input signal and the characteristics of the device. The interaction between the input and the device is described by a mathematical infinite integral called convolution. The output is the input convolved with the impulse response of the device. The spectrum of the output of a device is simply the spectrum of the input multiplied by the frequency response of the device via the FFT.
(2) The modulation of one audio file by another. For example, the use of a hand-clap echo sample could be convolved with a guitar chord sample to produce an echo effect which sounds like it was produced by the guitar.