You may see this promotional text section at the Store Listing on portal. If you are still wondering what it is then this article may help you to figure it out.
Actually, this promotional text is only needed for Apple. Your app’s promotional text will appear at the top of your app description. It contains up to 170 characters long. You could type everything in the promotional text. The gist of your current issues, your issue headline, and maybe your magazine tagline will work just fine. Also, you could consider using promotional text to share the latest news about your app, such as limited-time sales, upcoming features or content, or other events within your app.
You can always update promotional text at any time without having to submit a new version of your app. Go to your iTunes Connect account then choose the app. In your left hand section, go to the App Store then click on your iOS App.
Scroll down until you find the Promotional Text section then type it in the box. After you finish, don't forget to click on the Save button at the top right corner. There you go! Your promotional text will be displayed automatically.