 convolution is where one signal is slid over another, with each point on the stationary signal being multiplied by each point on the sliding signal as the slide occurs. This produces a third signal, the convolved signal. Convolution is often used to calculate how a system will react to some stimulus. For example, how an audio filter reacts to the sound of my voice. The stationary signal is the stimulus, my voice. And the sliding signal is the impulse response of the filter that is how the filter reacts to some sudden sharp impulse like banging something with a hammer. Convolution can also be used on two-dimensional signals like images to locate objects within that image. For example, where I am within this crowd of people. The Fourier transform uses convolution to locate specific sinusoids within signals.