One artistic representation among many.
Yggdrasil is... well, that's rather complicated.
What is Yggdrasil?[]
Most know Yggdrasil as the computer that runs all of creation, also known as the multiverse. It is also known as the great tree upon which universes and multiversal clusters grow like branches. Other interpretations paint it as a living brain which essentially 'imagines' all of reality.
And the most honest answer you'll get out of an Administrator if you ask them which one Yggdrasil is would be "yes".
The Infinite Loops[]
Even the Admins aren't entirely certain what happened to Yggdrasil that necessitated the last-ditch fail-safe known as the Infinite Loops. All that is known is that it is the only catastrophe of its level to ever hit Yggdrasil in divine memory.
Yggdrasil's capabilities, while still infinite from most dimensional perspectives, are now greatly diminished. At least 90% of what is left is dedicated to the prevention of another such incident as what necessitated the Loops, and most of what's left is tied up trying to keep the myriad of viruses, malware, and bugs that used to be minor issues from mucking up what's left of reality.
Running Reality[]
Trying to operate Yggdrasil may sound simple when admins try to explain it to mortals as best as they are capable of, but the reality is ludicrously complicated. A mortal soul trying to perceive the coding of Yggdrasil would be like a single typed character trying to perceive the program it is a part of. That is, categorically impossible in every sense of the concept.
To put it into perspective, admins have likened operating Yggdrasil to "sitting down to a massive multiplayer social luck-based strategical roleplaying, world-building board game halfway through where the pieces, the dice, and the cards all had their own ideas about how to play and even the board itself wanted in on the action"[1].
The Greatest Existence?[]
It is logical that Yggdrasil is at least on the same dimensional plane as the admins (generally agreed to be 10D+), but some suspect that it may be an even greater dimensional existence than they are (11D+). All that can be said for certain is that...
Yggdrasil... Is.