This is a Hint 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.
This is a rectangular box with a indent pointing left on the left hand side. On the outside top and bottom of the box are 3 lines that radiate out at different angles. Inside of the box are three lines representing text.