An instrument’s  dimming curve is simply how bright the light is in relation to the level of the associated DMX channel.

A good dimmer should be perceived by your eye to be smooth and linear: if you set a light level of 50%, the lamp should appear to be half as bright as it does at 100%.  Not all light sources are inherently linear like this.

For example, to make a LED look like it’s at half brightness, the power going to it must be reduced to 25%.  Correction for this is done with a dimmer curve.  For LEDs, the best curve is Inverse Square Law.  For tungsten lamps, a true linear curve is best.