Automation is the ability to dynamically control various aspects of a track's innate attributes and the attributes of any processors attached to it. In Ardour, automation can be used to make dynamic changes to a track's:
Any combination of these can be enabled on a single track; as such, it offers a lot of power and flexibility over how a track will ultimately sound when played back.
To activate automation for a track, click the A icon on the track head and check the type of automation you need.