Rendering 3D room interiors where most of the light comes through a glass window is difficult for most render engines. For technical reasons I won't bore you with, they tend to make very noisy grainy images which take ages to render properly. Portal lights are a special type of light designed to solve this slow rendering problem.
If you want a simple explanation; the more of the lighting which happens on the first bounce, the easier it is to render. So If a light directly hits an object, it will render cleanly and quickly. The more the light bounces and bends before it reaches its destination, the slower the rendering will be. So for sunlight coming in through a window, the light bends once when it enters the glass, again when it leaves the glass, and so its only the third bounce of light which actually reaches the room. For the light which hits the ceiling, thats another bounce as it reflects off the floor, so the ceiling in the room will be very grainy.
To be honest, you probably won't need to worry about them until much later on, for now just stick with the simpler lights at the top of the light menu