When lawn aeration is basically a procedure that is done to a lawn by either a machine or hand tool that opens up the soil by removing cores of the soil or creates little holes into the soil to help promote drainage of water into the soil easier.
The core aeration of the soil also helps nutrients and oxygen to reach the roots of the grass much easier which helps promote a healthier lawn.
Overseeding a lawn is where you plant new grass seed over existing grass seed without tearing up the soil or turf and is used commonly to repair dead patches of grass.
So after you aerate the lawn you can overseed the lawn to help grow new and healthier grass on your lawn.