Rebirth

(from System -> Help -> Rebirth)

Upon completing Area 1-5, you will now be able to Rebirth. Rebirth is a prestige mechanic that provides you access to powerful upgrades that will increase your overall power and how quickly you are able to progress through the game. Each class has its own set of Rebirth points, meaning that to acquire all the necessary points to purchase some upgrades will require playing as all three classes and saving up those points!

It is recommended that you export your save before completing a Rebirth in case you decide to change your mind.

Super Rebirth
Super Rebirth is a rebirth where you can re-choose your class, but upgrades, EXP, and level aren't reset.