The Trans-Dimensional Portal is a form of technology that can open dimensional barriers allowing individuals to travel from one dimension to another with relative ease. the Kraang use it to travel to Earth from Dimension X.
The portal is first mentioned in "It Came From The Depths", and first shown in "TCRI". This portal is located on the top levels of the main TCRI building. Leatherhead was able to sabotage the portal by stealing and hiding the power cell that fueled it. He later entrusted the Power Cell to the Turtles, but the Kraang eventually stole it back and got the portal working again. When the Turtles and Leatherhead broke into TCRI to destroy the portal, they were unable to stop the Kraang from using it to bring Traag to Earth. In the end, Leatherhead was forced to pull Traag back through the portal with him to prevent anything else from coming through, and the Turtles were forced to escape TCRI before they could destroy the portal. The design of the portal vaguely mimics the Utrom Transmat.
In Metalhead Rewired it was revealed the Kraang have fast-traveling points all over NYC, which can be used to get to the prison they were keeping the mutants they captured from various locations in NYC. Some of these fast-traveling points are located in a vending machine, a port-a-potty and inside a mailbox.
In The Manhattan Project, Part 1 the Kraang are seen using portable portals, when the turtles used one of those they were sent to what donnie referred to as the Grand Central Station of trans-dimensional travel, a space between dimensions with doors leading to places like Dimension X, the 80's turtles dimension and others. In the same episode a variant of the original trans-dimensional portal was being used for bringing Kraathatrogons from Dimension X.