Impedance is the measure of the resistance of the loudspeaker at different frequencies and should ideally be the same at all frequencies.
Smooth, even impedance means better working conditions for the amplifier, resulting in higher resolution. The job of the amplifier and loudspeaker is to reproduce the signal exactly as it is, adding nothing and removing nothing.
If the impedance of a loudspeaker changes at different frequencies, the amplifier encounters constantly varying resistance, making it much harder to follow the signal/music.
Linear impedance - a stable load for the amplifier - is a high priority at DALI and is achieved by fine-tuning every aspect of the speaker design.