Ionization energy is like taking candy from a baby. If a baby REALLY wants that candy, it will hold on tight and it will take someone else a lot of energy to take it away. If the baby doesn't want the candy that much, it won't take much energy for someone to take it away.
Lets say the BABY=ATOM and the CANDY=ELECTRON
An atom has a HIGH ionization energy if it takes a lot of energy to remove an electron. An atom has a LOW ionization energy if its easy to take an electron away.