Orb of Augmentation

How to get these:

You can buy an Orb of Augmentation from an NPC for 4 Orbs of Transmutation.
You can get an Orb of Augmentation by selling some Rare items to an NPC.
By the end game these will be so common loot drops, that it will not even be worth picking up all of them

Orbs of Augmentation are common and inexpensive. Feel free to use them as you like while leveling.
Magic items can have up to two mods - a prefix and a suffix. Some mods (like +XX life) are always prefixes and some others (like +XX% resistances) are always suffixes.
If a Magic item only has either the prefix or the suffix, you can add the other one using an Orb of Augmentation.
Flasks don't have the Rare rarity. After you upgrade one to Magic rarity using an Orb of Transmutation and it is missing a mod, you can add the other one using an Orb of Augmentation.
A longer endgame crafting process often involves using many Orbs of Alteration to reroll the mods of a Magic item, adding the missing mod with an Orb of Augmentation if required. Later you can use a Regal Orb to upgrade it to Rare.