Destiny 2 banning issue - what are overlays?

[Quartz]

Apparently Bungie are being overzealous with banning Destiny 2 players. The issue appears to be 'overlays'. What exactly are overlays in this context?

I mean, I've used Afterburner to monitor GPU usage and tweak my GPU. Does it have another use? Will Task Manager be counted as an overlay now it has a GPU monitor?

MMOs aren't my cup of tea, so I'm not likely to be affected by this instance but who knows what I might purchase in the future?

 

[Quartz]

Given what I've read elsewhere, I simply don't believe them.

 

[Quartz]

They are banning cheaters and key hackers or people who bought from key hackers.

Let me elaborate: I don't believe that they're deliberately banning these people. Rather, there's something gone awry in their banning mechanism. I.E. I'm believing it's incompetence rather than malice.