Subdivison off
Subdivison On

Subdivision divides each triangle in the mesh into 4 new triangles, with each subdivision level increasing the number of triangles exponentially. Therefore a large mesh of, say, 1 million triangles becomes 4 million triangles when subdivided only once. When subdivided twice it becomes 16 million triangles, and when subdivided three times it becomes 64 million triangles.

Below are available settings:

View Dependent Subdivide only meshes visible by the camera
Max Subdivisions How many times to subdivide the mesh. Polycount becomes exponentially larger with each subdivision level.
Curvature Threshold Triangles will not be subdivided if their curvature is smaller than this threshold.
Pixel Threshold Triangles will not be subdivided if they are smaller than this pixel threshold.