It is the letters "H1" in capital letters. The letters are only outlined, they are empty on the inside. The "H" looks like a normal capital H, and the "1" is the same height as the "H" with the top section pointed diagonally towards the "H".
Related to Header 1 Icon
Related icons are the icons with matching tags, as well as all editing icons. All icons are in the same flat style. You can either download free PNG icons or buy SVG vectors