It's an icon of a number one enclosed in a circle. The number is circled, possibly to indicate that a list will follow or a question may follow the icon. The icon is entirely black and white.
This is a 1st 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 13,300 icons, all in the same style and quality.