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 can change the Text and Background colours, and various Typography settings like Font size, Appearance, Letter case, Line height & Letter spacing. You can also add an HTML Anchor to the heading which enables you to create a link to a specific part of your page.