F-Droid is the most well-known "F" in the Android world. It is a community-maintained repository that exclusively hosts free and open-source software (FOSS).
F-Droid explicitly flags apps that include advertising, user tracking, or dependence on non-free services, putting the user's choice first.
The term typically refers to a modified version of an existing application. In the context of a "Pro" version, this usually means a developer or third-party has unlocked premium features that are normally behind a paywall. F-Droid for Android - Download the APK from Uptodown