When you cut off one hydra’s head, two will grow in its place. This is not a group of German bad guys who plan world domination in Captain America, or a dragon-like creature from Greek mythology that has a lot of heads.
Hydra is a critter, a living being that pretty much has the same traits we’ve heard about in the movies. It can grow back the tentacles when they are chopped off.
Scientists took a hundred of hydras and chopped them into individual cells and clumped them together. They formed ball-like cells and formed a completely new hydra.
It is said that hydra has a life hack for itself. One scientist once isolated a hydra for four years and it was just fine. It has surprisingly long lifespan for a creature that small. Later on it was discovered that it shed its cells and was building its body again from shreds.