"Yggdrasil, the tree of the world. Guardian of wisdom... and fate also." ― Johann Schmidt
Yggdrasil is a figurative tree used by the Asgardians to describe the cosmic nimbus that connects the Nine Realms. Each branch connects to one of the realms.
An Yggdrasil representation at the Hall of Science
displaying the  Nine Realms
1942, when the HYDRA leader Johann Schmidt invaded the village of Tønsberg, Norway, he found the Tesseract hidden in the wall with a carving of the Yggdrasil Tree on it.
More than sixty years later,
Thor explains to Jane Foster the concept of the Nine Realms, by drawing Yggdrasil in a sketch pad.
The tree in the Hall of Science is connected to Yggdrasil and indicates the health and well-being of the Nine Realms. If Ragnarök happens, the branches which represent the affected realms would fall off.