Complex sentence: The Fock states form a complete set of basis of the many-body Hilbert space, known as the Fock space, such that any generic quantum many-body state can be expressed as a linear combination of Fock states.
Split into simpler sentences: The Fock states form a complete set of basis of the many-body Hilbert space, known as the Fock space. A generic quantum many-body state is a linear combination of Fock states.