كِتَٰبٌ فُصِّلَتۡ ءَايَٰتُهُۥ قُرۡءَانًا عَرَبِيًّا لِّقَوۡمٍ يَعۡلَمُونَ
A Book whose ayahs have been detailed, an Arabic Qur’an for a people who know.
The word فُصِّلَتۡ comes from a root that carries the meaning of separating things from one another and making each part clearly understood.
When someone speaks very fast and you cannot follow them, you say, “Slow down, explain it piece by piece.” When dealing with a complex matter, the way to make the subject understandable is to take one element and separate it from another, and then from the next, and so grasp how everything fits together.
This is the method of explanation that the concept of fussilat offers.
While tafsir means explanation in general, fussilat sets out a particular methodology of explanation: it refers to breaking matters down into parts and addressing each one individually, so as to leave no room for any ambiguity or confusion.
Allah states that the ayahs have been detailed and the matters addressed one by one, in order to provide complete clarity.