Ionization energy is the energy needed to remove one electron from an atom in the gaseous state.
This electron would be a valence electron, or an electron in the outermost energy level/shell, because they're the easiest to remove.
Ionization energy depends mainly on the strength of the attraction between the negative electron and the positive nucleus.
When we move down a group in the periodic table, more energy levels are added, and so valence electrons would become further and further away from the positive nucleus.
This causes the attraction between valence electrons and the nucleus to decrease, something known as the shielding effect.
The less attraction between the electrons and the nucleus, the easier they are to remove—decreasing ionization energy.