In its simplest terms, hi-res audio tends to refer to music files that have a higher sampling frequency and/or bit depth than CD, which is specified at 16-bit/44.1kHz.
In 2014, the Digital Entertainment Group, Consumer Electronics Association and The Recording Academy, together with record labels, formally defined high-resolution audio as “lossless audio that is capable of reproducing the full range of sound from recordings that have been mastered from better than CD quality music sources'.
Unlike high-definition video, there’s no single universal standard for hi-res audio.