There is a Wikipedia article for this location.
Mimer's Realm is a single map in northeast Midgard. It is connected to the northeastern Forest area to the west(if you can bypass the bordering mountains), River Vid to the north(if you can bypass the mountains or quicksand), Jotenheim to the south, and the Dungeon (10 meters, northeast) through a portal (purple in Valhalla, red in Ragnarok). It is also one of the locations reachable by dimension traveling to yellow(the other being Azare's Plane).
A player's main interest here's the quest for Heimdall's horn, Gjall, obtained by defeating Aspenth down the well.