Augments are extra stat sticks that equip to your second mercenary slot and are not useable in PvP or BvB. They're buyable in two tiers, Tier 1 (Crystal x Giga cost) and Tier 2 (Crystal x Coin cost). Augments greatly boost individual stats of your choosing, however they are only available in 1, 3 and 7 days - so they are very much a lategame crystal sink item. Here are the available Augments -

Permanent Augments

This augment takes the second merc slot and gives an array of powerful bonuses. Comes in two tiers, Tier 1 gives +10% EXP and other stats, Tier 2 gives +20% EXP and other stats.