It is a circle with a horizontal line directly in the middle of the circle. The horizontal line stops slightly before reaching the edge of the circle so no part of the line actually touches the circle itself.
This Minus 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 5,700 icons, all in the same style and quality.