A transmatter portal is a construct that is capable of instantaneous travel between any two points in the universe.
Roulette began working with Maaldorian slavers to kidnap Human subjects. They used a transmatter portal located under a fake health clinic to transport humans from Earth to Slavers' Moon in order to sell them into the galactic slave market. Kara Danvers and Mon-El investigated a string of missing persons and discovered their plot. They both went through the portal to rescue the kidnapped humans. When Alex Danvers led a D.E.O. strikeforce to the portal, J'onn J'onzz recognized that it was a transmatter portal and tasked Agent Winn Schott to get the portal working. He activated it allowing Alex to lead the D.E.O. on a rescue mission to Slavers' Moon. When they returned with the rescued human captives, Winn reactivated the portal allowing everyone back to Earth.
The D.E.O. kept the Maaldorians' portal on Earth and later reconfigured it to transport them onto Lar Gand and Rhea's spaceship. J'onn, Kara, and Winn used it to rescue Mon-El and used it again to transport themselves back down to Earth.
Rhea went to Lena Luthor with a proposal to build their own transmatter portal. Lena thought the portal's possible benefits would greatly help the Earth and Rhea claimed that she wanted to use it to return back home. Lena and L-Corp built a giant prototype outside National City but there were having problems stabilizing it. Whenever they turned it on, it caused Phorians in National City to go berserk and unleash their psychic powers out on everything and everyone around them. When Lena managed to stabilize the power, Rhea revealed that the portal was being used to bring all the scattered Daxamite ships across the universe to Earth. Supergirl tried to destroy it but it was self-sufficient and had a barrier to protect it as hundreds of ships came through it into National City.