|MadSci Network: Astronomy|
This is an interesting question. I'm not sure I understand exactly what you mean, but I will try to answer it.
The answer has to do with where the light is coming from. We see things because they either give off light or reflect light. When you look outside in the day, there is a lot of sunlight, and the sunlight is reflecting off many things (like the leaves on the trees). All this sunlight and reflected light comes through the curtains, allowing you to see outside.
Inside your house, there is not much light being given off or reflected. The light bulbs in your house aren't very bright compared to the sun. So very little light is coming out through the curtains compared to how much light is already outside. Therefore, people can't see in very well.
If it were dark out and you had your lights on inside, things would be different. Since it would be brighter inside, more light would be going out through the curtains and people outside could see you inside. But you couldn't see the people outside, because they would be reflecting very little light.
I hope that answers your question!
Try the links in the MadSci Library for more information on Astronomy.