The phrase "sun in the sky" is a simple and common expression that refers to the presence of the sun in the sky or, more specifically, to its visibility. It is a way of saying that the sun is shining, which means it's a clear day with plenty of light. The phrase can also be used to describe a good or happy mood, as in "I'm feeling great today - the sun is in the sky and I'm ready to take on whatever comes my way."