An impulse signal for a digital path is a waveform that is completely at neutral besides the first sample, which is at maximum amplitude.
By feeding such an impulse signal into a completely digital chain (such as a reverb plugin), an impulse response file can be generated. This impulse response can be used by a convolution plugin to re-create that digital chain.
A simple Max patch can be used to generate the impulse signal. This results in a very small (4KB) wave file, which can be loaded into a digital system and fed into a digital effects chain, as demonstrated below. The output can then be rendered, resulting in an impulse response file.
Once the impulse response file has been rendered, the impulse response file can be loaded into a convolution plugin.