Core items explanation:

Pick one of the 2 most left items on each row, depending on the damage type your enemy have that disturbs you during each part of the game.
The prices of the item in each row fits the duration in the game you will get to that item for a specific role (support, initiator/tank, carry).

Then take the most right item at each row (most cases a must item, unless specified else on the item).

Itemization in general do 4 things:
1) cover for things the hero lacks (life steal, crit, stun, etc...).
2) compliment the hero type (str/agi/int).
3) negate the main damage type of the enemy (physical/magical).
4) affordability - item should be affordable at the right times of the game according to the hero role (support, tank, carry).