Second ionization energy is the energy needed to remove a second electron from an atom after one has already been removed. It always takes energy to remove electrons from atoms, although the amount of energy varies greatly. This process is known as ionization, which makes charged ions from neutral atoms.
The amount of energy required for each ionization increases, so the second ionization energy is always greater than the first ionization energy. This is because it is harder to remove an electron from a positively charged ion than a neutral atom. Some elements like sodium lose their first electrons with only a little energy but require vastly more energy for the second ionization. Others have a more steady progression of ionization energies.