The Heading Block allows you to insert an HTML Heading into your page. By default, the heading will be a H2 but you can change this using the icons on the Heading Block toolbar.
Once you've added your text to your block, there are further options available in the Settings Sidebar. You change the HTML level for the heading (i.e. H1…H6) along with the alignment for your heading (Left, Centered or Right aligned).