A Portal is piecing together the 'level' or 'map' into viewable portals for the engine to pre-render to speed up the game.
Example:
8====================8============11======8
The = is a corridor, the 8's are doors and 11 is a crossroads. There will be a portal between 8 and 8, 8 and 11, 11 and 8.
When you are in the portal 8 and 8, it won't draw (or extensively draw) portal 11 and 8.
It kinda works on that premise, however there are multiple ways an engine can render and optimise 'portals' it's down to programmer preference. OpenGL engines can't really have 'portals' as it is supposed to be able to draw near enough the entire map from any given view (regardless of what is blocking the view). FPSC is in DX so rendering and portal building can be as contained as defined in the engine.
