In practice, convolution is being able to apply the sound of speaker cabinets or reverb from real spaces to a dry sound source. This is done by creating an impulse response. These are
captured by generating a known sound signal in an environment, eg. through a guitar speaker cabinet or in a large room. The difference between the dry signal and the recorded signal
is able to be calculated and used in a way that it can be applied to other dry sound sources. Eg. an impulse response captured in a church could be applied to a dry vocal recording to
make it sound like they were recorded in that church. Another common application is capturing impulse responses from speaker cabinets, so you can make a guitar or bass sound like
they are being played through them.
Convolutions plugins such as IR:LV2 and Klangfalter can load up .wav impulse responses and apply them to whatever sound source you want. Guitarix also has a built in convolution
plugin called convolver, which can be used in combination with speaker cabinet impulses.
Impulse response files can be found on various forums on the internet.
IR:LV2 running reverb impulses in Ardour