This is a very simple icon that is meant to represent the plus sign used in addition. It's made of two lines of exactly the same size crossed over each other so that they run perpendicular. The lines are fairly thick.
This Plus Math 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.