Meaning: This sentence simply states that the sun gives us light and heat during the day.
Choice of Words: The sun is a big, bright star in the sky. It gives us light and heat. "Shine" means to give off light. "Brightly" means with a lot of light. "Daytime" is the time between sunrise and sunset. By saying "sun shines brightly in daytime", we describe the sun giving off a lot of light during the day.