Definition
A specific regulation in Title 14 of the Code of Federal Regulations, Part 61 (Certification: Pilots, Flight Instructors, and Ground Instructors), Section 61.31, paragraph (f)(1). This paragraph sets out the requirement that, with limited exceptions, no person may act as pilot in command of a tailwheel airplane unless that person has received and logged flight training from an authorized instructor and has received an endorsement in their logbook attesting to proficiency in tailwheel operations.
Plain English
A federal rule that says, with a few exceptions, you cannot fly a tailwheel airplane as the pilot in command unless an instructor has trained you in one and signed your logbook to confirm you can handle it.
Context Anchor
Seen in training material about high-performance airplanes, logbook endorsements, and pilot qualification requirements.
Derivation
CFR stands for Code of Federal Regulations, the official compilation of U.S. federal rules. Title 14 covers Aeronautics and Space. Part 61 deals with pilot certification. The numbering after the section (the letter and number in parentheses) drills down to the exact paragraph being referenced -- in this case, paragraph (f), item (1).
Why Pilots Care
Pilots must satisfy this requirement to operate high-performance airplanes legally and to reduce the chance of accidents from insufficient experience with high-speed or high-power aircraft.
Intuition Check
Do not read this as just a page reference. It points to a legal requirement: before acting as pilot in command of a high-performance airplane, the pilot must meet the training and endorsement rule it names.
Example Sentence 1
Before her first solo in the Citabria, the instructor signed the logbook endorsement required by 14 CFR part 61, section 61.31(f)(1).
Example Sentence 2
Reviewing 14 CFR part 61, section 61.31(f)(1) showed that the additional training applied only to airplanes with more than 200 horsepower.