ABC/HR is an abbreviation of ABC/Hidden Reference.
It is a method to 'score' the quality of audio encodings by comparing them alongside a reference sample. The 'hidden' comes from the fact that the listener does not know which sample -- the left or the right one -- is the reference.
The reference used usually is a sample of higher quality than the tested samples, e.g. uncompressed/losslessly compressed audio tracks, or a lossily compressed track with higher bitrate.