It's an exclamation point drawn inside of an equilateral triangle. The triangle is drawn with one side parallel to the ground, and the exclamation point is in the center of the triangle.
This is a Error icon. We've made it in iOS style
, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 13,700 icons, all in the same style and quality.