An abbreviation of
High Fidelity.
Hi-Fi is, by definition, reproduction (of sound) as close to the original recording as possible. This means flat frequency response, high power (actually high SPL), and low distortion.
Valve amps have much higher distortion than modern solid state amplifiers, and very low output power. Some people enjoy the tube sound very much, and there is nothing wrong with that — but calling it High Fidelity is a misnomer.