The description block is part of Airtable Blocks, a Pro plan feature. Blocks let you extend the functionality of your bases: you can use blocks to bring new information into Airtable, visualize and summarize your records in colorful ways, and even directly integrate your Airtable bases with your favorite apps.
The description block gives you the ability to write rich text notes or descriptions for other blocks or your entire base. Use it to provide additional context for your base, or create documentation for your fellow team members to get them up to speed.
The description block has a rich text editor that allows you to format your text in a variety of useful ways. You can provide headers, bold/italicize/underline text, create numbered or bulleted lists, and turn text into links.