Diving deep into the study for PMP you will definitely realize the importance of measurement be it the performance of the project or the project manager or the project team. Typically there is a baseline used for measurement and the deviations around it need to be addressed. The triple constraints - time, cost and scope each have a baseline which is a part of the Project Management Plan. Ofcourse all of this is done during the planning phase.
Now these three baselines put together is known as the Performance Measurement Baseline. Mathematically speaking, Performance Measurement Baseline = Scope Baseline + Schedule Baseline + Cost Baseline But do note that while Schedule and Cost baselines are just the final versions of the agreed upon schedule and cost, scope baseline consist of the following, Scope Baseline = Project Scope Statement + Work Breakdown Structure + WBS Dictionary Once the Performance Measurement Baseline is established, agreed upon and signed off by the relevant stakeholders, it becomes the ultimate benchmark of measuring the project and all discussions of keeping the project on track surrounds the PMB