What does HDR mean in Mac Photos?

It means the photo was shot wit High Dynamic Range.

HDR mode is sort of a method to make photos have less contrasted exposure differences (at least how I understand it).

For example, if there's a scene you want to shoot that has a really bright area (maybe the sun reflecting off a window) and also a dark area or two (an alleyway or shaded porch), a normal shot would expose for one of the extremes, depending on your choice. Exposing for the dark area would leave the light one washed out. Exposing the light area would leave the dark ones too dark to see.

HDR does its best to expose for all light levels in the shot at the same time, so hopefully you will be able to see the things on the dark porch, but also the details surrounding that reflective window.

Wikipedia: https://en.wikipedia.org/wiki/High-dynamic-range_imaging

Honestly, though, this is something easily searchable. I typed HDR into Google to make sure I was getting it right, and the first result was that Wikipedia page. It probably took more effort to ask that question that it would've to type the three letters into a search engine.

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