What is parsing?

[Naaura]

No. It's quite easy to tell when player(s) are underperforming with a quick glance at information available in-game. What is this thread anyway?

I would like to know how one can tell if others are underperforming if you can tell how. I personally cant tell unless I can visually see that they are distracted and just not doing their rotation and following boss mechanics.
And this thread is about what is parsing because I kept hearing for years but I finally remembered to ask since it was mentioned earlier