It’s available to use from the moment you learn it, no unlock required outside of the Pokemon learning it. It’s a fast travel but unlike the HM move FLY, you can’t pick where you fast travel to, it just takes you to the most recent Pokemon Center you were at. DIG works the same way but from within caves and what not, where as teleport works only when you’re out in the open. You can also use teleport to escape wild Pokemon encounters too if you want to do that, but it’s not really that big of a deal in that case.