Let's say you have a poem program, that reads files from your drive and turns them into poems. A well isolated/abstracted variant of that program is as simple as a blackbox with two or three inputs and a single output.
One of the inputs are the files, the others might be a configuration file or user adjustable parameters like length. The program is well isolated if you can't give it any combination of inputs that doesn't produce a poem or an error message related to the usage of the program.
A badly isolated variant of the same program would be one where the user had to think a lot about the internal behavior of the program, e.g. how file names are handled or where so many parameters of the poem generation have to be supplied as parameters, that the user essentially has to rewrite the core of program with their parameters. Or the user could supply a file that allows them to gain RCE or crash the program.
I don't think it is anything to do with complexity, or grouping code/data, its just a natural tendency of people to categorize things together that display a high degree of class inclusion. And some categories are easier to deal with than others.
