# How does the Active Space Transform work theoretically?

I have a question about the ActiveSpaceTransformer which is used in vqe calculations for a molecule of LiH in Qiskit. In the documentation the inactive Fock operator is defined.I don't understand why is the inactive Fock operator defined in that way, what is his physical meaning? Then, Why are we substituting the one body integral with the inactive Fock operator in the original hamiltonian?