Complacency is checking the box.
Here is a slice of cheese between two slices of toast.
"You asked for a grilled cheese, didn't you?
Not even close.
Complacency is doing the least and switching the auto-pilot back on.
Complacency is insiduous. After a while, we can't even tell what is good and what is passable any more.
Here is an example.
There is a Thai restaurant in my town. It was the only one for a long time. Anyone who wanted any coconut curry went to that restaurant. And they loved it. Only when a competitor opened a new Thai restaurant in January did most people start to realize they have been missing out on some good curry.
The first Thai restaurant checked the box for so long that it defined the box. No one could tell how a Tom Kha soup should taste when prepared well. Since most of its customers did not typically travel to broaden their taste-horizons, this restaurant was free to set the bar as low as it wanted. And what was passable food at best was seen as good food.
You can either be unhappy with passable choices, or be complacent.