When equipment drops, it can roll a quality of + or ++. You'll see it at the end of the item's name, like Truth + or Truth ++. A + item shimmers silver and a ++ item shimmers gold.
Each + adds 2 Item Power and boosts one of the item's stats, like its damage, rate of fire or health. A ++ item gets two boosts. Boosted numbers on the tooltip are marked with (+) or (++).
Rarer items are more likely to roll quality:
Shiny items are always ++.
Starter items (0 Item Power) never roll quality.
Quality can only boost something the item already has. For example, a weapon only gets a leech boost if it already leeches, and armor only gets a Wisdom boost if it already gives Wisdom. The item's affix counts too.
Like affixes, a boost is bigger on a low IP item. Each item's page lists its own boosts. Here's what one + can give:
You can change an item's quality at Oyun's Rerolling Cauldron in the Tavern. See the Rerolling Guide for prices.
Reroll a +: Changes what that + boosts.
Same quality duplicate: A chance to add a +: 1 in 8 for most items, 1 in 4 for Special, Unique and Event items, and 1 in 3 for Relics. Each try fills the upgrade bar, and a full bar is a guaranteed upgrade.
Higher quality duplicate: A guaranteed +.
Lower quality duplicate: No upgrade, but the reroll is still free.
Quality can't go higher than ++.