In this context, it can refer to participants, interventions (treatments, tests), comparisons (differences from other treatments or tests), and outcomes (what is expected to be accomplished)
P refers to Participant, which means the participant, which is often referred to as the patient or subject in a clinical trial. Intervention, which is the treatment or preventive measure applied to a patient; C is Comparison, which means control, referring to a control group in a clinical trial or another intervention that can be used for comparison; and O is Outcome, which means ending, usually referring to clinical efficacy of a treatment.
Since it is a principle, it is of course a brief formulation, and when applying this principle to the choice of treatment options for a specific disease, the issues that are sorted out vary considerably from one disease to another, and a great deal of specialized knowledge is required to make sense of the sorted out issues.