Lights are, as already mentioned simply points in space which emit light, but they are also very important and, because of the fact that they are simple to understand, lights are an excellent training entities.
To start, make a light entity by right clicking somewhere on the 2D grid and then clicking on light. A dialog should appear which asks you for the light intensity, and, by clicking OK there, you have set your first property on an entity, you have now given the light, a property called light, with a value of 300 (if you didn't change the default value in the dialog).
You can now press N to open the Entity Window and you will see that there is now an entry called light added. There are also two other values which the editor automatically fills in, namely the classname which simply tells what kind of entity this is, and the origin which defines the position of the light in your map.
So lets make the light flicker, and it's time to add another property, to the light; the style property. In the information window of the entity window you should see an explanation of the property, so, you should see that to make a flickering light, you have to add the property names style and give it a value of either 1 or 6. You should also try to look up some other values in the information window, and add them, to see their effect.