Portals are designated areas that tell the renderer where, in regards to interiors, environment light enters the scene. The smaller an opening is, the more important portals become. Without portals, the renderer has to shoot rays in all directions. That means that the chance to hit the opening in the wall is rather poor and this results in more noise (this is technically not exactly what's happening, but it's a useful approximation).
A simple example of the use of portals
It is important to note that most renderers support some way to define portals. However, the details of how to make them and how they work differ greatly. It is of utter importance to check the details in your particular software of choice on how the portals work.
Portals must be placed in a way that they cover the opening completely. If there is a gap between the portal and, for example, the window frame, some GI rays may hit the bright environment outside and produce noise. This can be even worse than not using portals at all, so make sure you cover the opening properly.
Portals are not the same thing as area lights. The color or texture of an area light is fixed to the specific point on the area whereas a portal passes through the information behind it. A "point" on that portal can have all sorts of colors depending on the angle you look at it. You can think of it as the difference between a painting and a window.
The disadvantages of portals
- An interior may have hundreds of small openings which are very hard to cover sufficiently
- Using hundreds of portals can be slower and less efficient than not using portals at all (dependent on the renderer)
- Placement mistakes can lead to noise that is hard to identify
- Portals may be subject to secondary ray clamping which makes lighting a scene with small openings a lot more difficult
- Portals may not support importance sampling of the background (like an HDRI) which can be a lot noisier and slower than using a dome light which supports it
- Some implementations of portals (or dependent on settings) ignore geometry behind it, so there might be no contribution of bounce light behind a window
- Some renderers do not support irregularly shaped portals (think of an S-shaped opening). Using portals, in this case, may be very inefficient
- It can be more efficient, depending on the renderer and scene, to use a dome light instead of portals