Hi-Res Audio refers to digital audio files with sample rates higher than CD-quality (above 44.1 kHz/16-bit), typically 96 kHz/24-bit or 192 kHz/24-bit, capturing more nuance.
CD quality: 44.1 kHz / 16-bit (the redbook standard). Hi-Res standard: 96 kHz / 24-bit or higher. Hi-Res Lossless (Apple Music): up to 192 kHz / 24-bit. DSD (Direct Stream Digital): used in SACD, very high sample rate (2.8 MHz+).
Most listeners can't distinguish Hi-Res from a 320 kbps MP3 on consumer headphones in blind tests. Benefits show on premium DACs + high-impedance headphones in quiet environments.
Services: Apple Music (free Hi-Res), Tidal HiFi Plus, Qobuz, Amazon Music HD.