The Fit to Fly test is a Polymerise Chain Reaction (PCR) test for the presence of SARS-CoV-2 (COVID-19 PCR antigen test).
Several countries have mandated Covid-19 RT-PCR or Antigen test at least 72 hours before travel for international flights, while several others have mandated self-certification by travellers that they are Covid negative. The Fit to Fly test when produced prior to your flight, or at the Port of Entry (destination) allows you to travel or even be exempted from quarantine upon entering a country in certain cases.