Portals are wounds or ruptures in space-time and the fabric of reality itself, bridging the disconnect between dimensions.
In 1983, various portals began opening in and around Hawkins, Indiana, connecting the town to a mysterious alternate dimension, known and referred to only as the Upside Down. The first portal was opened in Hawkins National Laboratory's underground subsystem, following an experiment gone wrong. A predatory creature native to the Upside Down could create additional portals of its own, which it used to leapfrog in and out of dimensions. These portals were temporary, 'healing' and sealing up some time after their creation.
The first portal was opened at Hawkins National Laboratory. During an experiment, Eleven made psychic contact with the Monster, causing a wall to crack open and form this portal. Unlike the other portals, this portal didn't heal and close - in fact, it only grew in size, slowly spreading and unfurling in the lab. Dustin, Mike, Lucas and Eleven would later refer to this portal as the Gate.
The Monster also possessed the ability to create portals. These portals were essentially dimensional "wounds" that would eventually "heal." The amount of time it took for these portals to close seemed to vary, as it took quite some time for the portal in Joyce's wall to heal. While the portal the Monster used to get away from Jonathan and Nancy seemed to vanish almost automatically.