There's a half circle with rays beaming off it that looks like the sun going behind a flat line. Above the sun is an arrow pointing down giving the idea of a sunset.
This Sunset icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 6,000 icons, all in the same style and quality.