Portals sample more efficiently environment lights used in interior scenes. Portal lights are helpers placed manually to let the renderer know where light is coming from. Inside an interior scene, they are typically placed where the windows are. When placed correctly, render time savings can be substantial as the renderer knows where light is coming from. For the same amount of samples and render time you get way less noise.
Portal gizmo in 3D View
To use Portals, add them to the Environment Light Portals list and turn on the Enable Portals checkbox: