Plain English Meaning
It means stopping to ask, 'Am I actually fit to fly this flight, right now?' before committing to it. You're checking your own body, mind, recent practice, and stress level the same way you'd check the airplane.
Full Definition
In the 5P checklist used in Single-Pilot Resource Management (SRM), 'the Pilot' is the first of the five P's and refers to an honest self-assessment of the pilot's own fitness to fly at that moment. It covers physical readiness, mental state, recent flight experience, currency, proficiency, fatigue, illness, stress, medications, and .