This is hashtag. there are two parallel lines on top of another two parallel vertical lines, like what you would see on various social media websites.
This Hashtag 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.