ما معنى طبقات السند؟

The layers of the isnad (chain of transmission) refer to its collective narrators, with each narrator representing a layer. For example, "Malik from Nafi‘ from Ibn ‘Umar" consists of three layers. The importance of this becomes evident in the mutawatir (concurrently transmitted) hadith, where it is stipulated that each layer must be composed of a group of people whose conspiracy to lie is inconceivable.