The clouds, the air, and the earth's surface all absorb and reflect radiation from the sun. Weather satellites fly around the earth high above the atmosphere. Their visible light sensors can "see" sunlight that is reflected from the earth and clouds. Since visible imagery is produced by reflected sunlight, it is only available during daylight hours.
When visible images are taken during sunrise or sunset, you can see a dark shadow across the part of the earth where the sun has already set.
Visible satellite images look like black and white pictures. Clouds usually appear white, while land and water surfaces appear in shades of gray or black.