― Neo deadpans, staring at mountains, after instantly traveling between Le Vrai and the Chateau[src]
A Portal is a doorway that serves as a a special shortcut through the Matrix, allowing for instantaneous travel between two locations even though they may be many miles apart. Portals are accessed using special keys, without which they will simply appear as ordinary doorways.
Portals were first introduced in The Matrix Reloaded, with The Merovingian, Persephone, and the other Exiles using them to pass between Le Vrai restaurant in the middle of the Mega City and the Merovingian's Chateau in the mountains miles outside of it. The Keymaker is an expert at making portals.
Portals are similar to the Backdoor space (also introduced in The Matrix Reloaded), but while portals allow direct access between two points, the "programmer backdoors" are an endless hallway allowing access to many other portals.
By the time of The Matrix Resurrections, the resistance creates portals that lead into trains, because moving portals are harder to track. They then exit through portals in different doors on the train - somewhat like how the backdoor hallway leads to many other portals, but these are located on actual trains in the main construct of the Matrix.