Basically the Noon Sakinah is a Noon (ن) devoid of any vowels; namely the Kasra (كسرة), fatha (فتحة), and dammah (ضمة). In the Quran, the noon Sakinah can be depicted with a Sukoon (سكون) above it or without anything above it at all like this (ن). It is important to note that both have the same pronunciation and are called Noon Sakinah.
But the tanween is an extra Noon Sakinah used with the Kasra (كسرة), fatha (فتحة), and dammah (ضمة).
If you are interested to listen to the pronunciation of the noon sakinah and tanween take a moment to read this article: Learn to read Quran with Tajweed Rules: Noon Sakinah and Tanween
Diacritics are usually forgotten; however, they are extremely important in order to learn Arabic pronunciation. Diacritics (تشكيل), pronounced as Tashkil, literally means Forming, which is its main function, since Arabic letters on their own, without Tashkil, do not show proper guidance as to the proper Arabic pronunciation. Therefore, learning diacritics goes hand in hand with learning Arabic.
If your interested to learn more check this article: Learn Arabic Diacritics: Tashkil