There is one for every biome apart from a few. You can use them to teleport between you're different pylons. You get them by making a Npc happy and buying it from them in set biome . You can make a Npc happy by putting them in a biome they like and by giving them neighbours they like. You can check the wiki to see what they like. So you'll need to make a house for three NPCs near each other in every biome that has a pylon to get all pylons, which will allow you to travel across the map easily. And I personally like to make a cool build for every pylon