This information can be considered as quest spoiling. Open at your own risk!
An astral bridge is used to travel from one spot to another. To travel with an astral bridge you need to carry an Orichalcum Pearl which will be destroyed during the transport. These can be bought from any representative of the Explorer Society for 80 gold each. They look similar to ordinary white pearls.
The Explorer's Society provides two astral bridges for explorers who has finished the last mission of the Explorer rank in Explorer Society Quest. One is between Port Hope and Northport (near Carlin) and the other is between Liberty Bay and Svargrond.
In the Knightwatch Tower you can find astral bridges to Ab'Dendriel, Ankrahmun, Carlin, Darashia, Dream Realm, Fibula, Kazordoon, Mintwallin and Thais. These are only provided after you joined a faction in the Dreamer's Challenge Quest and for some you need to be promoted to higher rank in your faction.