Cloudless means bright and clear, like the blue, cloudless sky you're hoping for on the day of the big back-to-school picnic.
When you look up and don't see a single cloud, you can describe the sky as cloudless. Cloudless skies mean no rain, no haze, and no fog. In the daytime, a cloudless sky is blue and bright, and a cloudless night sky is perfect for stargazing through a telescope. This adjective is formed by adding the suffix -less, "lacking," to cloud.