In a normal jellyfish life cycle, the jellyfish will transform from an immature polyp stage to a sexually mature medusa stage. After reproducing, the jellyfish will die. However, the turritopsis nutricula can revert back to its polyp stage after achieving maturity. In laboratories, the turritopsis nutricula has been able to continue doing this indefinitely.
This would be like an adult human turning back into a little kid, then growing up again, over and over again. The turritopsis nutricula can’t die of old age, so it earns the title of “immortal”. Of course, unlike the normal definition of immortal, the “immortal jellyfish” can still be eaten or die from natural causes.