Portals are ruptures in the fabric of reality, bridging the disconnect between dimensions. The main portal to the Upside Down dimension was located at Hawkins National Laboratory, however the Monster could also create temporary portals, from which it could leapfrog in and out of dimensions.
The first portal was opened at Hawkins National Laboratory. During an experiment, Eleven made 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.