Teleport plates are warp/fast travel points normally found on the ground throughout the world. The plates allow you to quickly move across large chunks of the game world by simply choosing an place with an activated plate in the menu's map (ESC). The teleport plates are where you will get teleported to when you choose your destination on the map, and step through the portal.
You can only teleport to a destination if you have previously walked over it to activated it. The teleportation plate in Evergrind City and the teleportation plate for the Flying Fortress gets activated as soon as you become a collector and place the teleportation plate down respectivley, (probably to ensure that players can leave and return to the Flying Fortress) but all the other plates require activation.
After a teleportation plate has been activated it gets added to your map, indicated by a yellow ring around a green dot.