Idea of Header 1 Icon
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".
This Header 1 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 6,200 icons, all in the same style and quality.