When someone is fucking up and their superior notices that they are fucking up, said person is put on notice. This notice, which is essentially a warning, reminds the person that their superior can fire them, punish them, etc. Thus they are "at will" of the superior.
Tina doesn't do shit at work. She just sits around, surfs the web web, and talks to Rob. If she gets caught, she's going to be at will.