Why scientists care
Ionization energy is the energy required to remove an electron from a gaseous atom or ion in a specified state. Successive ionization energies refer to successive electron removals.
Its periodic pattern is powerful but not perfectly smooth because subshell structure, pairing and shielding matter.
A concrete example
Removing the second electron from Na+ requires vastly more energy than removing sodium's first valence electron.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.