HDR stands for High Dynamic Range and refers to a class of techniques for capturing or rendering a wider range of brightness values than a standard image format can hold. A standard (sometimes called SDR) image has 8 bits per color channel — enough for 256 levels of brightness — which is roughly the range a typical computer display can show. HDR images use 16 or 32 bits per channel and can represent brightness from black to direct-sun brightness without truncating.
In architectural rendering, HDR matters in two places. First, environment maps (the spherical images used to light a scene with realistic environmental light) are usually HDR — they need to capture the genuine intensity of the sun and sky to make the lighting feel real. Second, the rendering itself is often computed in HDR space and then 'tone-mapped' down to the standard display range so highlights don't clip and shadows don't crush.
AI rendering handles HDR internally — the model produces output that's been effectively tone-mapped from a much wider dynamic range than you see. The visible result is rich, balanced lighting where bright windows don't blow out and dark corners hold detail. You don't have to ask for HDR explicitly, but specifying 'naturalistic dynamic range' or 'preserved highlight and shadow detail' will push the AI to favor balanced output if it's leaning crushed or blown.
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