What is the Gunnah rule?

Understanding the Ghunnah Rule in Tajweed

The Ghunnah, in the context of tajweed, refers to a nasal sound that is produced exclusively from the nose, resembling a gentle 'hmm.' This sound has a specific length, which should not exceed two counts. Whenever you come across a Shaddah above the letters noon (ن) or meem (م), it is essential to produce the Ghunnah sound.