Indicates that Unity should not check index values when you use Mesh.SetIndexBufferData to modify a Mesh's data.
When you use Mesh.SetIndexBufferData to modify a Mesh's data, Unity's default behaviour is to validate the indices array that you supplied, to check for out-of-bounds index values. Unity throws an exception if it finds any.
You can make Unity skip these checks by using the MeshUpdateFlags.DontValidateIndices flag. This can be beneficial to performance, however if you use this flag you must make sure that you pass valid data to the Mesh.
Additional resources: Mesh.SetIndexBufferData.