Complex sentence: Rus-M is a rocket that is intended to become the launch vehicle for the Prospective Piloted Transport System, Russia's new manned spacecraft which is to replace the Soyuz.
Split into simpler sentences: Rus-M is a planned rocket which is intended to become Russia's main launch vehicle for manned spaceflight in the future. It will be the launcher for the Prospective Piloted Transport System, a new manned spacecraft which is to replace the Soyuz.