Concept art
The Transmat is an Utrom invention similar to Professor Honeycutt's Teleportal in the 2003 TV series It teleported the Turtles to another world, and eventually was used some weeks later to pull them back to Earth along with the Fugitoid. As the National Guard investigated the TCRI building, the Utroms were forced to begin evacuating the premises through the Transmat. The aliens successfully returned to their homeworld, and the Turtles were able to use the device to escape the Shredder, teleporting themselves a safe distance. The Transmat was then destroyed in a massive implosion. It's revealed in the episode Secret Origins that the waste products from the Transmat are the Ooze that caused the Turtles' mutation.
Leatherhead tried to build one in the episode What a Croc, so he can travel to the Utrom homeworld, but it was destroyed after a battle with Baxter Stockman destabilized the area.