Automation is the process by which a parameter is recorded in real-time.
In the days before computer-based recording we had to record onto tape, and this presented us with a real problem: recordings were destructive insofar as once a dynamic or effect was used, it was also recorded to tape, and the process could not be reversed.
Today we can do pretty much what we like, because every process within the virtual domain can be reversed. The computer has replaced tape as the recording medium. More importantly, we can mix a song and create countless versions of the same mix, editing each version to taste with the advanced feature sets in all DAWs.
The beauty of automation is that we can alter any dynamic, effect, or global event and record those alterations in real-time. Additionally, we can re-edit those events at any point.
Most DAWs have automation lanes that can be assigned to automate any process, whether a plugin effect or instrument, or a global event such as volume, pan, and so on. If you want to apply automation to a track, all you need to do is use the Write Enable function on that track, record the automation event you select and then throw the track into Read Enable mode to play back the automation.
Extract taken from the book Beat Construction.