Dynamic range is a measure of the range of tones the sensor can capture. Cameras with low dynamic range will often show 'blown' highlights or blocked-in shadows. This test is carried out in controlled conditions using DxO hardware and analysis tools.
Dynamic range is measured in exposure values (EV). The higher the number the wider the range of brightness levels the camera can capture. This falls off with increasing ISO settings because the camera is having to amplify a weaker signal. Raw files capture a higher dynamic range because the image data is unprocessed.
Nikon D5 dynamic range charts
JPEG dynamic range analysis: With the exception of the upper values the D5 performs consistently for much of its sensitivity range, producing images with a wide range of tones.
Raw (converted to TIFF) dynamic range analysis: While the D5's raw file dynamic range is good, it is beaten by the D4S and 1DX Mark II indicating that these cameras capture a broader range of tones and that images stand-up to a bit more post-capture adjustment. DXO Analyzer was unable to analyse the raw files taken at the D5's upper sensitivity values.