Topology describes the type of face that a meshThe main graphics primitive of Unity. Meshes make up a large part of your 3D worlds. Unity supports triangulated or Quadrangulated polygon meshes. Nurbs, Nurms, Subdiv surfaces must be converted to polygons. More info
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A mesh’s topology defines the structure of the index buffer, which in turn describes how the vertex positions combine into faces. Each type of topology uses a different number of elements in the index array to define a single face.
Unity supports the following mesh topologies:
Note: The Points topology doesn’t create faces; instead, Unity renders a single point at each position. All other mesh topologies use more than one index to create either faces or edges.
In the Mesh class, you can get the topology with Mesh.GetTopology, and set it as a parameter of Mesh.SetIndices.
For more information on supported mesh topologies, see the documentation for the MeshTopology enum.
Note: You must convert any meshes that use other modelling techniques (such as NURBS or NURMS/Subdivision Surfaces modelling) into supported formats in your modelling software before you can use them in Unity.