Dynamic range is the span, usually measured in stops, between the darkest shadow detail and the brightest highlight detail a camera sensor can capture in a single exposure.
Dynamic range is the reason a scene that looks fine to your eye can be impossible to capture in one frame. The eye keeps adjusting as it moves around a view, so a bright window and a shadowed room both look detailed; a sensor takes one fixed reading and can only record a set span of stops, roughly 10 to 14 on current cameras, between the darkest and brightest detail it holds. When the scene's range exceeds that, something clips.
Knowing this tells a photographer when a single exposure is realistic and when it is not, and points to the fixes: wait for softer light that compresses the scene's range, add fill or a reflector to lift the shadows, use a graduated ND filter to hold back a bright sky, or bracket several exposures and blend them. It also explains why overcast days are easier to expose than sunny ones.
A photographer shoots inside a cathedral with bright stained-glass windows. Metering for the windows renders the stone interior almost black; metering for the interior blows the windows to white. The scene spans roughly 15 stops from the glass to the darkest corner, well beyond the camera's 13-stop range, so no single frame holds both.
The fix is a bracket of three exposures two stops apart, one for the windows, one for the mid-tones, one for the shadows, merged later into an image that keeps detail everywhere. On a heavily overcast day outside the same building, the scene might span only 7 stops, so one well-placed exposure captures the whole range with headroom at both ends. A spot-meter reading of the brightest and darkest important areas tells you the gap in stops before you commit.
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