The Heading block unsurprisingly produces a heading. The block supports heading levels H1, H2, H3, H4, H5 and H6. You should probably not use the H1 block, that is usually used for your page title automatically.
Most blog posts will either have no extra headings or one extra level. Longer copmples pages may have more.
Heading block are inserted in the same way as any other, clicking the plus at the top left of the window or the one that appear when you press the return key and create a new block. You can also type /Heading in a new block .
Here is a quick gif tour of the Heading Block toolbar