Idgham (الإِدْغَام) means inserting; technically it is a silent letter meeting a voweled letter so that they become one doubled letter of the second kind. Its six letters are gathered in يَرْمُلُون, and it is of two kinds:
Idgham bi-Ghunnah
In four letters gathered in يَنْمُو: Ya, Nun, Mim and Waw. With Nun and Mim it is Idgham Kamil (الإِدْغَام الكَامِل), since both the Nun and its quality disappear and the Ghunnah belongs to the letter merged into. With Waw and Ya it is Idgham Naqis (الإِدْغَام النَّاقِص), since the Nun disappears but its quality, the Ghunnah, remains.
Idgham bila Ghunnah
In two letters, Lam and Ra'. It is a complete merge in which both the Nun and the Ghunnah disappear.
Its condition and exceptions
Idgham requires two words. If a silent Nun and Waw or Ya occur in one word, Izhar Mutlaq (الإِظْهَار المُطْلَق) is required so the word is not confused with a doubled root. This happens in only four words: ﴿ٱلدُّنۡيَاۖالدُّنْيَاﵐ﴾ (Al-Baqarah 2:85), ﴿بُنۡيَٰنٞبُنْيَانٌ﴾ (As-Saf 61:4), ﴿قِنۡوَانٞقِنْوَانٌ﴾ (Al-An'am 6:99) and ﴿صِنۡوَانٞصِنْوَانٌ﴾ (Ar-Ra'd 13:4), as the poet says: "except within one word, as in dunya."
Also excepted in Hafs by way of ash-Shatibiyyah: the Nun in ﴿يسٓ ١يٰسٓ ١﴾ (Ya-Sin 36:1) ﴿وَٱلۡقُرۡءَانِ ٱلۡحَكِيمِ ٢وَالْقُرْاٰنِ الْحَكِيْمِ ٢﴾ (Ya-Sin 36:2) and in ﴿نٓۚ وَٱلۡقَلَمِنٓ وَالْقَلَمِ﴾ (Al-Qalam 68:1), which are read with Izhar; and the Nun in ﴿مَنۡۜ رَاقٖمَنْﶌ رَاقٍ﴾ (Al-Qiyamah 75:27), because of the Sakt (السَّكْت) on it.