What is mean by non-gaussianity in ICA? Why do we use in ICA? How is Non-Gaussianity is an important and essential principle in ICA estimation?

Following is the statement I found in a research paper. But I am not able to understand it. Please explain with help of an example and mathematical equations.

Separation of independent signals from their mixtures can be accomplished by making the linear signal transformation as non-Gaussian as possible. Non-Gaussianity is an important and essential principle in ICA estimation.