This is a About 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 12,500 icons, all in the same style and quality.
It is a speech bubble with the letter "i" in the center. The bubble is not an exact circle, but rather a horizontal oval. The letter "i" is lower case, and one would think that by seeing this icon you would be informed about a certain topic presented with it.