Ionization energy is the minimum amount of energy required to remove an electron from a neutral atom in its gaseous state.
Ionization Energies cheat sheet for homework
This refers specifically to the amount of energy required to remove one electron from each atom in one mole of atoms, creating 1+ ions.
This refers specifically to the amount of energy required to remove a second electron from each singly-charged positive ion in one mole of 1+ ions, creating 2+ ions.
It's a measure that reflects how strongly an atom can attract a pair of bonding electrons towards itself.