A well lit room can have a big positive impact on the video quality of your calls. If you have a bright light behind you pointing into your webcam then your face will appear dark and featureless to whoever you are calling. If the overall level of light in your room is low then your webcam will try and amplify the light to make the picture brighter and that will reduce the video quality. A well lit room is one where there is a good amount of ambient light and where you will not have any bright lights right behind you.