In short
Potential Driving Instructor — a trainee instructor authorised to charge for lessons while qualifying.
A Potential Driving Instructor (PDI) holds a trainee licence from the DVSA. They are still working through the three-part ADI qualification but are licensed to charge for lessons during the trainee period.
PDIs display a pink triangular badge in the windscreen. Once they pass all three parts, they upgrade to a green ADI badge.