The Surface Reconstructor node generates an implicit surface out of a cluster of particles. An implicit surface is a mathematical function that describes a surface (just like a formula describes the surface of a circle which you may remember from math class).

It is most of the time used to generate the surface description for a liquid body (and the according liquid particles). If you want to generate a polygon mesh out of the surface description you need to link the surface reconstructor to an implicit mesher.

The Surface Reconstructor is a post operator which is not evaluated during substeps but only at the end of the simulation pipeline. Furthermore it can be evaluated on the fly and therefore does not need to be cached because it doesn't need any information from previous frames.

Having the following nodes available in the scene will extent the quality of the Surface Reconstruction process or may even be required:

Isotropic
Volume Properties (density and iso-density in isotropic mode; optional)

Anisotropic
Volume Properties (Weighted position. optional)
Surface Properties (Neighbor anisotropy; required) or Thin Sheet (calculating anisotropy)