High & Low Dynamic Range
Very simply, dynamic range is the difference between the brightest and darkest areas of a scene you intend to photograph. This difference can be measured and we use stops of Light to gauge this difference in Dynamic range.
A High dynamic range, is when the range of light or luminosity of a scene being photographed is greater than the abilities of you camera. So you cannot find a single exposure that can capture the whole range of light.
A Low dynamic range, is when the range of light or luminosity of a scene being photographed falls when in the capabilities of your camera.
So central to this is actually knowing what the dynamic range is of the camera you are using.