I understood the meaning of the sentence literally but that's not enough to understand the process. Could you explain how that works? Thank you!
The answer is chemistry in this case.
In terms of Hydrogen and Helium the atmospheres of the gas giants have atmospheres that most likely didn't change much since the formation of the solar system. In heavier elements however the gas giants are enriched (compared to the sun's atmosphere, which we think is the most original gas still around).
Such a mixture of elements, given at the pressures and temperatures in Saturn's atmosphere (as well as in Jupiter's) will form large amounts of Ammonia naturally by equilibrium chemistry.
If then the amount of Ammonia reaches vapour saturation, clouds will form as a result.