The description app is part of Airtable Apps, a Pro plan feature. Apps let you extend the functionality of your bases: you can use apps 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 app gives you the ability to write rich text notes or descriptions for other apps or your entire base. Use it to provide additional context for your base, store high-level checklists, or create documentation for your fellow team members to get them up to speed.
The description app 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 lists, bulleted lists, or checklists, and turn text into links.