Portals are found (in the Metroid series) only in Metroid Prime 2: Echoes, where they'll transport you from the light world of Aether to the dark dimension, and obviously vice versa. In the video game, two different types of portals can be found, each doing the same thing - the ones created by living beings, and the ones that were created naturally by dimensional breaches.
These gateways to the opposing dimension were first found after the Phazon meteor struck the planet. Soon enough, monstrous looking creatures started to leak out, causing fear in the Luminoth. However, these creatures would ultimately just evaporate from the world's light. Though soon enough they started to find ways of getting through to the light world without being killed, and thus a war ensued.
They were originally built by the Luminoth throughout the war, and can be activated if someone, notably Samus Aran, scans them. After this, you'll then be able to go through the portal into the contrary world of dark, or if you happen to be in the dark world - you'll be able to go to the light world.
"Dark Portal's allow living beings to travel to Dark Aether without being disrupted and scattered throughout the interdimensional void. They are unstable, however, and only remaining viable for a limited periods of time. Some can be reactivated, while others vanish forever once they are used."