Traversing, assuming you have the quest item and are not dead, allows you to travel from one plane of existence to the next.
For example, wherever you are, if you click traverse, a new section will popup allowing you to select the plane of existence you wish to go to.
If you then select the location and click traverse, you will be taken directly to that plane with a 10 seconds timeout.
If you do not have the appropriate item, you will be told you cannot traverse
There is no way to attain these quest items with real world money. You want to be able to traverse to new planes of existence, earn the right.
All traversing needs an appropriate quest item to travel from one plane to the next. For example, to get to the Labyrinth Plane, you need the Key to The Labyrinth.
Once you have it, from anywhere on the surface plane, you can use traverse to go to that plane. Planes in Tlessa, offer harder creatures, more adventures, more chances at items, gold, new places to settle kingdoms and so on.
The surface plane can be traversed to from any plane that is not the surface plane without a quest item.