Hi-resolution audio, especially for streaming. The general idea is that listening to digital audio files that have a greater bit depth and sample rate than CD (24-bit/192Khz vs 16-bit/44.1 KHz) translates to better-sounding audio, but in practice that isn't the case.

For a detailed breakdown as to why, there's a great explanation here. But in summary, the format for CDs was so chosen because it covers enough depth and range to cover the full spectrum of human hearing.

So while "hi-res" audio does contain a lot more information (which, incidentally, means it uses up significantly more data/storage space and costs more money), our ears aren't capable of hearing it in the first place. Certain people may try to argue otherwise based on their own subjective experience, but to that I say "the placebo effect is a helluva drug."