Jars are pickup items in Shovel Knight which restores the protagonist's ability to use secondary items. They vary with each campaign.
In Shovel of Hope, Shovel Knight can pick up magic jars to restore his magic, which is used to activate Relics. Small jars restore 5 magic whist the large ones restore 30 magic. Magic jars are found commonly throughout the game by defeating enemies, breaking blocks or at specific spots in levels. They are the only items that will continue to drop after Shovel Knight dies and continues a level.
The Conjurer's Coat makes enemies drop jars more frequently.
In Plague of Shadows, Plague Knight can pick up power jars to restore his power, which can be used to cast Arcana. They appear throughout the game in the same way magic jars do. Small jars restore half a portion of Plague Knight's power while large one restore two portions. Unlike magic, power regenerates over time.
In Specter of Torment, Specter Knight can find darkness jars to restore his darkness, which is used to power Curios. They appear throughout the game but also drop and float towards Specter Knight when he defeats enemies or hurts bosses. Purple candles in the Tower of Fate also generates darkness jars.