Plan quality management is the process where you need to define the quality requirements, standards, and guidelines. such standards and guidelines are the direction how quality will be managed throughout the project (i.e. in the control quality and perform quality assurance processes).
so in the plan quality management, you can define the metrics that you will use to measure quality (ex: defects per 100 unit), how such measures will be collected (criteria to consider a unit defected ), the control thresholds (ex: if defects per 100 unit is between 1 and 3, the process is ok, otherwise, actions will be taken), etc.
and it is during the control quality where you will collect the measures and compare them to the thresholds defined during the planning phase.
hope this helps