Plain English Meaning
Being genuinely good at flying, not just legally allowed to fly. It means your skills are sharp because you practice and stay current, not just because your logbook says you are.
Full Definition
The ongoing skill, knowledge, and currency a pilot maintains to operate an aircraft safely and competently. Proficiency goes beyond meeting the legal minimums for currency; it is the actual ability to perform flight tasks accurately and confidently under realistic conditions. Flight instructors are expected to maintain their own proficiency and to help students develop habits that keep them proficient throughout their flying careers.