Reincarnation is an action made available to players who reach their natural level cap, which starts at level 60.
When one reincarnates by clicking the reincarnate button in the reincarnation menu, their skill levels are reduced to zero, their upgrades are forfeited, and their level is brought down to zero. In exchange, their level cap is increased by 1, both their Myst and XP receive permanent 5% boosts, and they gain 10 permanent levels in the skill that was highest when they reincarnated. Some players opt never to reincarnate and instead level up using the upgrade system.
Neither method of leveling is inherently better than the other-reincarnation requires a tremendous amount but leaves your ceiling much higher while upgrades require less time but give you a finite level ceiling.