Planes of Tlessa is all about exploring new planes of existence, getting new gear, items, loot and fighting harder monsters.
To do that you need to first find the appropriate quest item, with that, you can then use the traverse, under the map - see traverse - to then traverse to the plane of existence, the item, allows you to.
With every other type of item in Tlessa, if you want to be able to traverse to other planes of existence, you first have to find the appropriate quest item.
You cannot buy the item with real world money.
Below are a list of planes, clicking on them will show you more information including how to access said plane.