What does it mean for a element to have a large ionization energy?

It means that element does not readily react with other elements. The lower the ionization energy the more reactive the element is.

Sodium has a low ionization energy therefore it is extremely reactive. Chlorine has a higher ionization energy. It is not as reactive as sodium.

Ionization energy is really the measure of the difficulty of removing an electron, or the strength by which an electron is bound. Therefore the higher or greater the ionization energy, the more difficult it is to remove an electron.