Meaning: Balloons move through the sky in a joyful and carefree way.
Choice of Words: Balloons are light, round objects filled with air or helium. Float means to move through the air without touching the ground. Happily means in a joyful and carefree way. In means surrounded by something. Sky is the big, blue thing above us. The sentence says that light, round objects filled with air or helium move through the air without touching the ground in a joyful and carefree way in the big, blue thing above us.