I'm assuming that you know the shielding effect is the lessening of attraction of the protons in the nucleus for an electron in the outermost orbitals of an atom. All the negative electrons below the outermost electrons tend to diminish the attraction of the positive protons. Since cesium's outermost electron (6s1) is so much farther away from the nucleus than sodium (3s1) electron, all the inner electrons shield it from the attraction of the nucleus.