The Wall is the barrier between the land of the living and the land of the dead, or the land of light and the shadowlands.
The shadowlands have a strong call and their own strange beauty, and only those with great ties to the world of the living can travel to the Wall and return again. The closer a person gets to The Wall, the stronger the desire to cross it into the shadowlands. Traveling to and from the Wall requires great energy, and few can do it without exhausting themselves. It is easier to travel to The Wall if a person goes there to call someone back instead of just going there to see it. The Wall can be approached in dreams, but even then it holds great power.
Calling someone back from the Wall requires great power. Elves and healers are able to travel to and from the Wall to call back a person about to cross the Wall, though once a person has crossed the Wall that person cannot return. Even though elves can approach the Wall, most prefer not to as it takes great effort (as they are not really supposed to be there: elves do not pass over the Wall when they die but instead go to the halls of waiting). It is part of a healer's test to go to the Wall. As seen in Book 2: The Horn of Moran, not all healers choose to go there. Alex is the only person known to have stood next to the Wall and returned.
While near the Wall, the power of great magic is evidenced, and those being rescued can see it. Loved ones or others of those approaching the Wall may be seen on the other side of it, though only rescuers (such as wizards) can speak with them and carry messages back and forth. Those who are rescued from the Wall have their memories blocked until the rescuer opens them.
The Wall also serves another purpose. It serves as the divide, the ultimate prison of the evils of the past. If the Wall were damaged enough to create an opening, souls from land of the dead could escape into the world of the living. There is only one tool that is known to be able to damage the wall, the Axe of Sundering. During Alex's battle with Zain, Zain was able to create a crack in the wall using the Axe and escape to the land of the living. (Alex subsequently destroyed Zain and repaired the breach.) Afterwards, Salinor, king of the Dragons, appointed Alex as Guardian of the Wall.