Quoting from the README file:
Library that write OpenFOAM cases as HDF5 archives instead of the default one-file-per-process-per-timestep-per-variable approach. This saves a lot of files, makes it easier to manage, copy, and post-process the results. An XDMF file is used to describe the contents of the HDF5-file and this can easily be opened in ParaView, VisIt or any other common postprocessor tool. The IO part is handled by MPI-IO, which makes it effective on clusters and high-performance computers with thousands of nodes and parallel file systems.