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.