According to ctor in dolfin/mesh/Mesh.h
/// Create mesh defined by Constructive Solid Geometry (CSG)
///
/// *Arguments*
/// geometry (_CSGGeometry_)
/// The CSG geometry
/// resolution (std::size_t)
/// An integer specifying the mesh resolution
Mesh(boost::shared_ptr<const CSGGeometry> geometry,
std::size_t resolution);
it is an integer specifying the mesh resolution. For more specific meaning you need to look into dolfin/mesh/Mesh.cpp
and possibly other implementation files.