Automation is essentially recording and playing back any changes made to the controls within a DAW or other recording device. you can write/read automation of things such as volume, pan position, or for that matter just about any value within a DAW. You can use it to make changes within plugins, synths, you name it and it can probably automated. Say you have a track and you want the vocal to have a slight volume boost during the chorus - you adjust the volume fader while playing the track or simply "draw" in the automation to the track - when played back it will perform the volume adjustment just as if you were physically riding the fader. It stores the automation in the track. With a control surface with motorized faders, you would see the fader moving with the automation you recorded.