As another comment says, it's "performance improvement plan": saying to the employee that they're not meeting expectations and they need to hit a set of goals or be fired. It's part of the documentation process used by companies to protect against liability: if they're going to fire you they want a paper trail of a nondiscriminatory reason why they did it. The "rosier" theoretical side is that a PIP is a wake-up call to a struggling employee that gives them a chance to improve.

Here the claim is that the manager was forced to put X number of people on PIP regardless, which would be under something like a stack ranking system where a manager isn't allowed to say all their employees are performing adequately, they have to choose some to put on the chopping block. Amazon is famous for doing this.

Performance improvement plan (or program). Typically the first step in a process to fire an employee for cause. It documents that as an employer|manager you've outlined a problem and created a plan that would allow the employee to meet expectations in the future.

Generally speaking, it should be a hint that its time to find a new job.

Tongue in cheek: paid interviewing period

Performance Improvement Program