The "round sun, moon, & rain" phrase typically refers to a Minecraft resource pack that replaces default, blocky sky elements with realistic, circular, and detailed textures. It aims to improve immersion for players, particularly when used alongside shaders for a more natural look.
This pack, often found on Planet Minecraft , improves immersion by replacing the default, blocky sun and moon with high-definition, round alternatives and refining rain textures. Minecraft DLC Marketplace: Texture Packs