Ionization energy = the amount of energy it takes for an atom to lose an electron
Remember that the nucleus is positively charged and the electrons are negatively charged. Because those opposite charges attract, the nucleus is pulling the electron towards it and so, the closer an electron is to the nucleus of an atom, the stronger it is being pulled.
Now remember that as more electrons are added to match the number of protons, the shells fill up and each new shell that is made to accommodate for the electrons is further away from the nucleus. That means that the more electrons are in an atom, the more shells there are, and as the shells get further away from the nucleus, the electrons in those shells are pulled with much less force than electrons in the closer shells.
Now all that's left to remember is that as you go down the groups, the elements have bigger numbers of protons and so they have more electrons and so more shells.
