Complex sentence: All the Fock states form a complete set of basis of the many-body Hilbert space, or 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: All the Fock states form a complete set of basis of the many-body Hilbert space, or the Fock space. Any generic quantum many-body state can be expressed as a linear combination of Fock states.