Down a group
Now read down a column instead of across a row.
Take group I: lithium 519, sodium 495, potassium 418. The values fall as you go down. Look at any other group and you will find the same thing.
The conclusion
Going from top to bottom down a group, the first ionization energy decreases.
Why it falls
Going down a group, the number of energy levels an atom has increases. The outermost electrons are therefore further from the nucleus.
The attraction the nucleus exerts on those outer electrons is weaker at that distance. A weaker hold is easier to break, so removing an electron becomes easier — and less energy is needed to do it.
The chain, in order
Further down the group → more energy levels → outer electrons further out → weaker pull from the nucleus → easier to remove → lower ionization energy.
Hold onto that chain of reasoning. The next two pages describe a different property, and it varies for very much the same reasons.