To add a custom description to your form header, simply click on Add form description in the form preview and then turn on Form Description. If you've already added a description and would like to edit it, click on the gear icon that's shown when hovering over the form header, which will take you to the Header section of the form settings.
A few notes about the form description
You can use the toolbar to format it.
This includes the ability to add formatted text (markdown support), different font sizes, links, and images. If you wish to add images, simply select the picture icon on the far right of the description toolbar.
Optionally, you can set the Description Source to Airtable field and choose an Airtable field under Description Field to have a dynamic description.
You can also choose to truncate the description, so long blocks of text need to expanded first. This can help decrease clutter while still providing useful information.