In my words: energy is needed to remove an electron from an atom, it increases as it moves left to right. And decreases as it moves down the column.

Ionization energy

Definition: The ionization energy, or ionization potential, is the energy required to completely remove an electron from a gaseous atom or ion. The closer and more tightly bound an electron is to the nucleus, the more difficult it will be to remove, and the higher its ionization energy will be.