A progression fantasy is a new term for a new(ish) genre. The term refers to a work of fantasy that focuses on a protagonist working constantly to gain an ongoing and ever-escalating progression of skills, power, and munchkinry. It is, in short, the gamification of narrative.
These stories include, but are not limited to, time loop stories, LitRPGs, rationalist fiction, and other internet fanfic; while there are certainly older examples -- Piers Anthony may have been an early pioneer, with his Bio of a Space Tyrant and Apprentice Adept series starting out with strong progression vibes -- modern progression fantasy double-downs on the progress, with humble origins building slowly and inexorably towards ever more power and complex goals... very much like a video game or RPG.
Progression fantasies include Mother of Learning, Worth the Candle, Blessed Time, and All the Skills, among many hundreds of others. I would also include Harry Potter and the Methods of Rationality, Harry Potter and the Natural 20, and probably Worm as progression fantasy, although they are not as fully focused on progression as are some works.