Ionization energy is the measure of how much energy is needed to remove an electron in an atom (or molecule) to form ions. The higher the ionization energy, the harder to remove the electron(s) in the atom. To better illustrate this, let me show you a similar example.
Sodium (Na) has an ionization energy of 495.8 kJ/mol
Helium (He) has an ionization energy of 2372 kJ/mol
Sodium is a very reactive metal. It normally reacts with water and produces an explosive reaction. Helium on the other hand is classified as inert gas. Inert gas does not react normally under standard atmospheric temperature and pressure.