Jacob’s ladder refers to refers to increasingly more accurate approximation for the exchange correlation functional. The easiest (and least useful) is to set XC = 0. This might represent the ground. The first rung of Jacob’s ladder represents a functional corresponding to a homogeneous electron distribution due to a homogeneous positive charge distribution (reminiscent of Thompson’s plum pudding model for the atom). Each additional rung keeps the terms of the previous approximations but adds correction terms. The nuances of each additional rung I am less confident about!