It is when you cook an egg and the yolk is facing upwards with the white around it. They are referred to as "Sunny-side up" eggs or "Juicy in the middle" eggs. It's just a way of cooking them.

Don't you think the yellow of the egg looks like the sun ? so when that side is up, it is called so. Look at these images : http://goo.gl/GJo5f This is how it looks like :)

Here are more terms describing how to cook eggs: over easy = an egg flipped and cooked on both sides but the yolk is still runny over medium = an egg flipped and cooked onboth sides but the yolk is a soft set consistency over hard = an egg flipped and cooked on both sides but the yolk is set. poached = an egg cooked in boiling water, and can also be made to different consistencies like the eggs above. hard boiled= an egg that's been boiled in water for 10 minutes and can be peeled and eaten without it breaking up.