If I remember correctly:

Personal improvement plan

Or

Performance improvement plan

Basically, it's a one on one meeting with a manager, who lays out what you're doing wrong or not doing. They talk with you about it or goals or whatever, and make a timeline that both of you agree of is fair.

The idea is that nobody is blindsided and expectations are communicated all around. It's actually great for everybody involved, but is also generally the first step to being fired, since a lot of shitty managers just see it as something they have to check off before pulling the trigger in firing you. Like a paper trail of why you suck.

So, "having a pip", or be "on (a) pip" just means managers are looking at you closely and recording your performance, and you're in danger. Not always resulting in being let go, but.... You know how these things go.