Cameras with more dynamic range allow you to take photos with dramatic differences in highlight and shadow areas while retaining detail in both.
Think of a brilliant sunset on a rocky beach: Bright sunset in the background, with dark rocks in the foreground. High dynamic range means more of the extremes will be faithfully reproduced.
Dynamic range test data courtesy of DxO Mark.77D test data on DxO Mark D3100 test data on DxO Mark
Very good image quality; Improved dynamic range at low ISOs; Very fast autofocus; Dual Pixel CMOS AF in Live View; Much improved RAW buffer depth; 1080/60p video; Quick Control Dial & top-panel LCD screen.
Default JPEGs slightly soft straight out of camera; Dynamic range & high ISOs lag behind competing cameras; No 4K video resolution.