What is an excitation signal in closed loop control?

Breifly my title is my actual question, but if you need nore info below is my full description.

**** The example here is Magnetic Levitation of a ball****

The system is unstable so a closed loop control is implemented to stabilize the system.

But in case of real time model identification open loop identification is not possible, so we need to implement a closed loop identification. This result in implementation of excitation signal.

** The info I read about excitation signal and I did not get it**

In closed loop identification the controller introduces a output noise and control signal correlation, which leads to model corruption. This correlation can be broken by introducing an additional excitation signal.()